SOMEONE was paying close attention to this summer's Chicago theater invasion. Source Theater's harrowing production of William Mastrosimone's "Extremities" is played with dangerously physical force, recalling the all-out acting style of the Steppenwolf and Wisdom Bridge companies.

The outrageous opening minutes of "Extremities" will leave audiences white- knuckled: While Marjorie is home puttering around the house, Raul walks in through the unlocked screen door. He's been watching the house for some time, knows her roommates are at work, and refuses to leave, taunting and terrifying her in her impotence. Raul throws her to the floor and attempts to rape her in full view of the audience. The sight and sound are doubly jarring in the close confines of the Main Stage.

With a shpritz of bug spray, Marjorie manages to subdue her attacker, and when the lights come back up, she's got him hogtied, blindfolded and caged in the fireplace. Enraged, Marjorie tortures him, dousing him with scalding water and ammonia, choking him on a tether. With each tug of the rope, her life becomes more inextricably bound up with his. Raul promises to come back and slice her up when he is freed, so she determines to do away with him herself.

With the arrival of the two roommates, Mastrosimone provides us with a distanced perspective. Raul desperately works their sympathies, arousing pity for himself and suspicion against Marjorie. Soon they -- and we -- are wondering just what kind of woman Marjorie is. (When we first see her, she's absent-mindedly torching an insect in an ashtray).

And since Marjorie has no witnesses and no evidence, she can't prove a thing in court. As she bluntly puts it, "Before they believe a woman in court she has to be dead on arrival." Mastrosimone caustically spins variations on the "she was asking for it" routine -- some of which come from the mortified roommates -- and the tables turn rapidly round and round in this living room war.

Director Dorothy Neumann has urged her actors take it to extremes. Steven Dawn, who injured his hand in rehearsals for the rape scene, is frightening in his ferocity as the coarse but cunning would-be rapist. Dawn's performance, alternating between wheedling almost-charm and raw-throated, red-faced shrieking, is a literal tour de force.

Jane Beard convinces both as victim and vengeful captor, going from fear-frayed nerves to clenched-teeth rage as she exacts her vicious eye-for-an-eye revenge.

Rounding out the cast with good performances are Katrina VanDuyn, as Terry, who does "what good little girls are supposed to do -- nothing"; and Barbara Klein, as Pat, a social worker and voice of reason, who at the height of the hysteria notes that "Marjorie seems to feel alienated."

EXTREMITIES -- At Sorce Theater Main Stage through November 2. "Extremities" contains brief nudity and extremely graphic violence and language that may be upsetting to some. Because of a cast member's injury, some performances have been canceled, but the show may resume Saturday, possibly with an understudy. Call 462-1073 for information.