Extremities Melodrama by William Mastrosimone. Starring Susan Sarandon. Directed by Robert Allan Ackerman.

With its numerous obscenities bleeped, William Mastrosimone's new crime thriller might serve the purposes of a sensational made-for-TV movie. Tautly written, staged, and acted, ''Extremities'' stars the exceptionally talented Susan Sarandon as the object of an attempted rape who manages to overcome her assailant and then savagely tortures him.

Thus the melodrama at the Cheryl Crawford Theater (Westside Arts Center) seeks to underscore the degree to which this particularly vicious crime debases both the violator and the one violated. The intention suffers from the fact that ''Extremities'' never rises above the level of headline sensationalism. The conclusion is maudlin.

In her intensely powerful performance as victim-turned-avenger, Miss Sarandon is strongly supported by James Russo as the manipulative would-be rapist and by Ellen Barkin and Deborah Hedwall as the heroine's devastated housemates. Marjorie Bradley Kellogg designed the simple living-room setting of the New Jersey house where the attempted crime takes place. The lighting is by Arden Fingerhut, the costumes by Robert Wodjewoski, and the violent action sequences by B. H. Barry. Scott Lehrer's sound effects include a between-scenes electronic whir that might suggest truck traffic along a nearby highway or a swarm of Jersey mosquitoes.

''Extremities'' is scheduled to move to Broadway in February.