On both sides of the street, signs advertise "Condominiums Available" and "Now Leasing Office Space." High-rise condominiums, towering office buildings and sleek mini-shopping centers with high-tech stores and trendy boutiques dot the landscape.

Until recently this stretch of Mass. Ave. between Harvard and Central Square was a self-contained neighborhood, with quiet restaurants and bars, its own movie theater and small, local businesses catering to the area's residents.

Since the real estate boom of the early 1980s, however, the condo developers and national chain stores have moved up from Harvard Square, slowly edging out the old way of life along Mass. Ave.

The pre-war apartment buildings are still there, and some small businesses still hold on to their leases, competing with the video and furniture stores for space. But while the old and the new seem to be peacefully coexisting, the neighborhood itself has taken on a different flavor, area business owners and community activists say.

"The whole character of Mass. Ave. is evolving," says Bobbi Bishop, director of marketing for Unihab Inc., which markets Bay Square Condominiums, one of the area's many new developments.

And while most people say that the changes are not for the worse, they add that the pace has quickened for this once-slow stretch between two of Cambridge's busiest areas. A new, high-tech way of life has moved in, with an upscale grocery store, a compact disc shop and futon vendors now lining the street.

Although Sally R. Alcorn of the Harvard Square Business Association says she doesn't think the neighborhood's character will be swallowed up, the number--and type--of new stores indicate the depth of the change.

Alcorn says that the area is still neighborhood-oriented. "Whenever you have a drugstore and a deli in the area, those are conveniences that residents are grateful for," she says.

It's just a question of what kind of conveniences residents need. Macaroni and cheese, no more. Now you can head over to Barsamian's to pick up a pound of Scottish smoked salmon for $26.50 or some rabbit pate for $9.58. Next stop: Videosmith, where local residents can drop off their Master-cards and rent the latest French film ($2.75 a day plus tax) to watch on their video cassette recorders.

But Alcorn says that the changes benefit everyone--big businesses and small, long-time residents and newcomers to the area. "The more life there is after 6 p.m. in a neighborhood, the better it is for the residents," she says.

And the more upscale, the better, according to store owners and residents.

"I don't see any opposition [to our new store] at all," says Marylou J. Sirois, a salesperson in the Furniture Store. "This is crafted furniture as opposed to a lot of mass-produced stuff," she says.

"The new stores are vastly less noisy and vastly better run than the [Orson Welles] theater was," says one resident who asked to remain anonymous. "The new stores are very sensitive to the needs of the neighborhood," she says, "in terms of noise, cleanliness, hygeine, safety and fire laws."

The pace of the neighborhood's development, although partially dictated by the booming real estate market of the early 1980s, has also been spurred by the evolution of Cambridge itself.

For example, when the Orson Welles Cinema, an independent movie theater located on the corner of Mass. Ave. and Dana St., burned down in the spring of 1986, it was eventually replaced by a host of new high-tech, upscale stores such as Videosmith, Digital Record, Toppers and The Furniture Store.

Ralph Hoagland, who owns the building that once housed the popular movie house, says that competition from USA Cinemas, the chain that has a near-monopoly on Boston-area theaters, prevented the rebuilding of the Orson Welles. "It was hard to get good product," Hoagland says.

Instead, he filled the building with service-oriented shops catering to the new Cambridge consumer. Each of the stores in the former Orson Welles building has at least one other shop in another location around Boston and Cambridge.

But as the new businesses take the place of the old, some small business owners say their establishments can still make a go of it along Mass. Ave., if they adjust to the changing times.

"Once they do a big building, only the big guys can go in," says Susan R. Phelps, co-owner of Hubba Hubba, a store that has been in Cambridge for nine years.

Hubba Hubba, which sells rock music, leather jackets and variety of novelty items, was forced to relocate two years ago in an effort to stay in the area. Now, a high-rise condominium development is being erected in Hubba Hubba's old home. "We moved with an eye to the future," Phelps says.

But while Phelps says small businesses can still survive along Mass. Ave., the new developments--such as the one which edged Hubba Hubba out of its seven-year location--are a "double-edged sword" for most small businesses.

On the one hand, the rents are going up, so it is. harder to find affordable space, she says. But at the same time, business is improving because "the new developments are bringing in people who have more money, people who are better educated," she says. "The alcoholic bums don't give me business."

Bay Square condominiums is the building which edged out Hubba Hubba. The condominiums, which are selling for $150,000 to $400,000 are being advertised in such upscale newspapers and magazines as Harvard Magazine, The Boston Globe and The Tab, Bishop says.

Real estate experts say that the new condominium developments such as Bay Square will definitely have an impact on the neighborhood's composition, but they add that it is not certain whether the condo market is strong enough to make the developments a financial success.

Bishop says, "Sales are taking longer than we had originally anticipated," but adds that she expects sales to pick up this month when customers will be able to see the units. As of late this fall, only 25 percent of the condominiums had been sold, she said.

But the problem is not the location of the new condos, according to one local expert. Rather, it is the state of the real estate market nationwide that is slowing down sales for the new Mass. Ave. units.

"These condominiums are representative of a city-wide pattern," says Joel Alstein, a principal of FARgroup, a Cambridge-based real estate development company. "We are in a glut economy when it comes to condominiums."

"It's not that the area is poor," Alstein says. "Traditionally, a condomiumium in middle Cambridge sells for more than an equivalent unit in East Cambridge or Cambridgeport," Alstein says. The area is very desirable, he says, "because the closer you get to Harvard, the better off you are."

The condominiums--and the stores that have accompanied the developments--were designed with a specific consumer group in mind, Alstein says. The developers have targeted their condos toward "empty nesters"--affluent couples who want to sell their houses in the suburbs after their children have gone off to college.

"But maybe they're not leaving their nests so quickly these days," Alstein says in offering an explanation about the sluggish sales of the new condominiums."

"There is a flatter market," he says, "but it is not dead at all." The market perks up in late spring or early summer because Cambridge operates on an academic year, he adds.

And while Alstein theorizes about the reasons for slow condo sales, the buildings continue springing up on both sides of Mass. Ave.

Across the street from the Bay Square development, a small frame house and a gas station have been replaced by 931 Mass. Ave., a tall, slate-grey condominium development.

"The selling has been fine," says Peter C. Kash '71, executive vice president of The Condominium Collaborative, the development's marketing company. Kash said late this fall that 29 out of 54 units had been sold.

But whatever the financial prospects for the area between Harvard and Central Squares, Harvard students and residents can't help but notice the changes.

One newcomer to the area--Shake, Rattle and Roll--seeks to capitalize on the Baby Boomers' nostalgia for 1950s memorabilia. Whether it succeeds in the market or not, perhaps the store's presence, located next to The Friendly Eating Place and down the street from the gleaming new condo complexes, is itself a metaphor for the changes that Mass. Ave. is undergoing.

Cajun food may come and go, the sushi market may bottom out--the face of Mass. Ave., though, is sure to be different.