An Evening of Improvisation. Presented by Peter Frisch's Freshman Acting Seminar. At the Loeb Ex, December 11-13, at 7:30 p.m. Tickets available free at the box office the day preceding each performance.

Do It Yourself. An original musical revue featuring short skits interspersed with songs, all written by Harvard and Radcliffe students. Sources agree that the talented six-person company does a dazzling job with some very funny material. A must-see, especially if you know of the of authors. In the Leverett House Junior Commom, December 11-13. Performances Thursday at 11:30 p.m., Friday and Saturday at 8 and 11:30 p.m.

Equus. The show to catch if you're up to leaving Harvard this weekend. Peter Shaffer's powerful play gets a fine production here, with superb acting by Dai Bradley as a boy who goes around blinding horses and Brian Bedford as the cynical psychiatrist who tries to cure him. At the W ilbur Theater, 252 Tremont Street, through January 10. Performances every evening at 8 p.m., matinees W ednesday and Saturday at 2 p.m.

Godspell. Stephen Schwartz's musical based on the Gospel according to St. Matthew, which you may remeber from its lavish and innovative film version a few years ago and the AM radio hit "Day by Day." At The Charles Playhouse, 74 Warrenton Street, through December 21. Performances Tuesday-Friday at 8 p.m., Saturday at 6:30 and 9:30 p.m., Sunday at 3 and 7:30 p.m.

Iolanthe Gilbert and Sullivan obviously had a good time satirizing the pomposity of the British House of Lords--the task inspired some of their cleverest lyrics and catchiest melodies. Iolanthe is one of the favorites of G&S aficonsdos--who vie with each other in attempts to render the tongue-twisting Nightmare song-- and the current G&S Society production makes it evident why.

As usual, the G&S Society has assembled an outstanding cast who combine comedic talents with operatic voices. W hat's unusual about this production, however, is that G&S Hall of Famer Tom Fuller has forsaken the stage-- where he's played heroes from Ralph Rackstraw to Nanki Poo-- to make his directorial debut. Fuller's production of Iolanthe lacks the inventiveness of past G&S shows like the gimmicky H.M.S. Pinafore of two years ago, but it features moments of comic brilliance rarely matched on the Harvard stage. The scene in which two Lords try to decide which will win the hand of fair young Phyllis is superbly executed, with Jeff Zax and Clifton Lewis playing off each other like pros. Dennis Crowley mades an equisitely tormented Lord Chancellor, Susan W . Van Colt and Douglas Morgan as the straight leads have a beautiful pair of voices, and Sallyu Stunkel plays the Queen of the fairies in a style pleasantly reminiscent of Glinda, the Good Witch in The W izard of Ox.

A really fine musical evening, all in all. If you can't get tickets, try ushering--it's cheaper and you'll probably get better seats anyway. At the Agassiz, December 11-13, at 8 p.m.

Miss Lonelyhearts. A partically successful production of an adaptation of Nathaneal W est's novella about a sort of grotesque Dear Abby. The play itself is a watered down, '50s version of the original, but Stephen Kolzak's direction is tight and his cast--with the unfortunate exception of Robert Beusamn as Miss Lonelyhearts--ably conveys the negativity of W est's vision. In the Quincy House Dining Room, December 11-13, at 8:15 p.m. Tickets $2.

Murder at the Boston Garden A comedy mystery about the adventures of a Boston private eye and part-time real estate agent who investigates a series of strange murders at the Boston Garden and the fate of a new basketball team called the Boston Ceramics. Sound pretty wierd. W ritten by Robin Brecker of the Boston Repetory Theatre and performed by the Rep in the Theatre in the Garage in Harvard Square, through mid-January. Shows W ednesday through Sunday at 8:08 p.m. Promptly.

Play it A gain, W oody Allen's comedy about the sexual hangups of poor Allan Felix, a schlemiel who spends most of his time daydreaming about Humphrey Bogart, his idol, and all the women he'd like to conquer. See Set Kaplan's review on page 2 of today's Crimson. In the W inthrop House Junior Common Room, December 12 and 13, at 8 p.m. Tickets $2.

Sir and William Penn. An original two-act play set in the 17th century England and subtitled "The Birthpangs of Religious Freedom." Another product of the Bicentennial. Performed by the Fisherman's Players in Old W est Church, 131 Cambridge Street, Boston. Shows Deceber 11, 12, 18, 19 at 8 p.m.

The Calvin Coolidge Follies A musical comedy revue about the career of our illustrious 30th President. At the Charles Playhouse Cabaret, 74 Warrenton Street. Performances Thursday and Sunday at 7:30 p.m., Friday and Saturday at 7 and 9 p.m.

Wonderful Town. The Loeb breathes musical life into an adaptation of the 1940s hit play My Sister Eileen about two sisters from Columbus, Ohio who invade New York to seek their fortunes. A melodic Leonard Bernstein score and an occasionally witty book are complemented in this production by fine performances from Rhonda Lee Goldenberg and Susan Terry, who look and sound just right as the two sisters. A good bet for nostalgia buffs and musical comedy lovers, who won't mind the formulaic plot. At the Loeb, December 11-13, at 8 p.m. Tickets $3 and $3.50.