TWO GIRLS, FAT AND THIN By Mary Gaitskill Poseidon Press. 304 pp. $18.95

"HOWARD ROARK laughed," begins Ayn Rand's novel The Fountainhead. But Rand herself wouldn't laugh if she'd lived to read Two Girls, Fat and Thin. Mary Gaitskill's first novel is, among other things, a ferocious send-up of Ayn Rand and the followers of her Objectivist movement. Gaitskill's fictional philosopher-novelist is named Anna Granite, a squat, plain woman whose novels have titles such as The Bulwark and contain turgid prose: "When I look at this stone, Miss Maconda, I see not only an object made up of mineral and material parts, but properties of color, curves and density that exist in their own point in space. This rock exists, Miss Maconda, because it exists."

In her first book, the acclaimed short story collection, Bad Behavior, Gaitskill wrote about men and women who live on the fringe. In Two Girls, Fat and Thin, she returns to that territory for deeper exploration. The novel is filled with outcasts; among the assortment of would-be philosophers and groupies surrounding Anna Granite is Dorothy Storm, an overweight, self-loathing young woman with a history of sexual abuse by her father. While in college, Dorothy becomes captivated first by Granite's novels and then by the woman herself, until finally she is drawn into the core of Granite's Definist movement, hired to transcribe lofty round-the-clock bull sessions. For the first time in her life, Dorothy feels important, but this is fleeting; after the movement is fractured by betrayal, Dorothy is eventually thrown back into her solitary life.

The action of Two Girls, Fat and Thin begins when Dorothy answers an ad in a newspaper seeking people who have had experiences with Definitism. The interview gives her a chance to reopen her past and tell her story to the interviewer, Justine Shade, a woman who is as tiny and attractive as Dorothy is large and ungainly. The two women's histories are counterpointed in alternating sections and, while the details of each are vastly different, both stories have much to do with cruelty: that of parent to child, child to child, lover to lover.

While Dorothy spent her childhood and adolescence as a victim, Justine once experimented with victimization herself, sexually terrorizing a passive girl from her class. The scene depicting Justine's aggression is handled adroitly, and Gaitskill strikes an appropriate tone of cool deliberation, filtered through a child's gaze: "Justine went on talking, saying that . . . everybody did this, didn't Rose know? . . . The torture feeling was roused and roaring as she wheedled and teased, moving closer and closer to the agitated, awkward kid until she was all but cornered against the wall, pulling hair across her lips, Justine whispering that Rose was a baby, a goody-goody, that she didn't know anything."

Several other flashbacks to childhood and adolescence in the novel are less successful. While Gaitskill is scrupulous with details, providing the interesting names of various neighbors (the "Sissels" and the "Kopeikins") and the titles of books the character read as children (My Father's Dragon, Little Witch and Peter Pan), the wealth of information finally becomes overwhelming.

Much more effective are Gaitskill's moments of brooding, powerful description. A girl is seen as "a Chinese puzzle of tension and beauty" and Anna Granite has "the crabbed, down-pulled mouth of a bitter old woman poking furiously around in a bargain bin for something she doesn't really want anyway." And in the final scene of the novel, an extremely graphic depiction of sadomasochistic sex, the strength and ludicity of description almost keep the reader distracted from the question of whether or not the scene is sensationalistic.

Two Girls, Fat and Thin continually tries to make points about the nature of violence and the easy escape-hatch of fantasy. Names such as Storm and Shade and Granite or Dr. Venus and Dr. Mars -- the therapists the two girls see as adolescents -- give Gaitskill's prose a heightened, florid quality. In this regard, the novel is reminiscent of the potboiler prose of Ayn Rand, and, we are meant to presume, Anna Granite. But Mary Gaitskill uses this tone to great effect; Two Girls, Fat and Thin is a deliberately overblown and demanding novel, imperfect in its excesses, but admirable in its weight. Meg Wolitzer is a writer living in New York. Her most recent novel is "This Is Your Life."