As thousands of college and prep school students descend on Cambridge looking for parties this weekend, more than 36,000 rowers from all over the world will arrive to participate in the 25th annual Head of the Charles Regatta.

While most Harvard students equate the Head with large swarms of strange people asking where they can buy beer, the regatta is actually a serious athletic event. The Head is the largest single-day rowing event in the world and the most famous in the United States.

More than 850 boats--with crews coming from as far the Netherlands and Lithuania--are entered in the 16 events slated for Sunday. And many past and future Olympic, world and national champions will be testing their skills on the Charles River.

Because of the number of turns and bridges the coxswains and rowers must negotiate successfully, the Head of the Charles is widely considered one of the most challenging of all the head-style races.

The first race begins at 8:30 a.m. and the last will end at 4:30 p.m. The course runs upstream three miles along the Charles from the Boston University Boathouse to the Herter Center in Brighton.

For Harvard and Radcliffe crews, the Head is a chance to match their strokes against some of the finest in the world and get a look at their racing technique before they go indoors for the winter months.

Coming off last May's Eastern Sprints championships, Harvard's heavyweight and lightweight crews--both consistent contenders for the national title--are looking to get a "Head"-start on next spring's season.

The men's heavies and lights are each entering a championship eight crew, a championship four, a club eight and a youth eight, but all eyes are fixed on the most important class of boats.

The stiffest collegiate competition for the varsity lightweight team will come from Yale--and Princeton should be a big concern as well. The lights have already defeated the Elis, overcoming the Yale crew by six seconds in the Head of Connecticut Regatta two weeks ago--and that was with some of Harvard's top rowers missing.

The Crimson is expected to outdo Yale by an even wider margin with its complete team rowing. A win would give the lights their third-straight collegiate championship in the Head of the Charles Regatta.

But don't bet the ranch on the Crimson winning the overall lightweight eight race just yet. There are still other non-collegiate crews they need to worry about, namely the U.S. and Canadian national teams.

Last year, the Canadians rowing in the St. Catherine boat the U.S. team and Harvard hope to give the Canadians a run for their money this year.

"A medal [for finishing in one of the top three places] would be great for us considering the two elite crews," said Harvard light-weight Captain James Baker.

The men's heavies also have their work cut out for themselves as they compete against two national teams, the American and British. In addition, the heavies are expected to have more trouble with their collegiate rivals since they must row against strong crews from Wisconsin, Navy and Northeastern.

"Our people are very excited," said heavyweight Captain John Bernstein. "Although a lot of our people graduated, we still have a lot of talent on the squad, and I think we'll do well on Sunday."