IN "TOOTSIE," which opens today, Dustin Hoffman portrays a woman. It's a preposterous proposition, but he brings it off convincingly.

In "Sophie's Choice," also in the theaters for Christmas, Meryl Streep digs down into her Vassar-Dartmouth-Yale Drama School experience to come up with Sophie Zawistowska, the voluptuous, doomed Auschwitz survivor first imagined by novelist William Styron. She says the characterization came easily; in any case, her reviews have been ecstatic.

In "Kramer vs. Kramer," Streep and Hoffman played opposite each other as Ted and Joanna Kramer. She was a mother who walked out on her child; he was a father who tried to become a mother. Their highly charged enactment of modern trial by marriage won each an Academy Award.

Now, both find themselves occupying the no man's land at the front of the post-Thurber battle of the sexes: Hoffman, as an aggressive male who says he's discovered a female counterpart within himself; Streep as a talented, ambitious woman, a feminist who nevertheless models some roles on men.

Both eschew the role of social model, but both enjoy working "close to the bone." "Acting is like life. It's only exciting when the sparks fly," says Streep. During "Kramer vs. Kramer," they were flint and steel. "Yes, it's true," says Hoffman. "I hated her off and on."

Dustin Hoffman became a star in 1967 at the age of 30 when an older woman named Mrs. Robinson took a fancy to him in "The Graduate." In 15 movies since, his roles have ranged from Ratso Rizzo of "Midnight Cowboy" to chic single in "John and Mary" to Jack Crabbe, the former Indian fighter who lived to 120 in "Little Big Man." Lately, his portrayals have taken an autobiographical turn. For Ted Kramer, Hoffman drew on his own recent divorce. Now, at age 45, he helped playwright Murray Schisgal develop the dual character of Michael Dorsey/Dorothy Michaels for "Tootsie." The film was originally titled "Shirley," after Hoffman told director Sydney Pollack he believed he'd always had a "Shirley" inside him.

Michael Dorsey is an actor who can't find work until he puts on a wig and girdle and successfully auditions for the role of a feisty woman administrator on a soap opera. Dorsey's transformation -- and apparently Hoffman's -- results in what, at times, seems an object lesson in feminist thought taught by a workaholic male actor. Hoffman makes it clear that he did experience a change.

"I do a lot of preparation for all my roles," he explained. "When I finally had Dorothy -- getting the voice right was the hardest part and took almost a year -- I could actually go out on the street and be taken as a woman. At that point it was like a dam had broken for me. I noticed immediately that men treated me differently. They were nicer, and I responded to it. So I wanted to be nicer for them. I'd be trying on clothes, looking in a mirror and suddenly it would strike me -- 'Hey, I look good in this.' I really wanted Dorothy to be good-looking. One of the big disappointments for me was that my nose is so big."

Like Michael Dorsey, Hoffman says, he developed a reputation as a "difficult" actor who got kicked out of acting school, walked out of parts and was fired from Off-Broadway, although he never actually tried becoming a woman. The closest he came was as Ted Kramer, a husband fighting to prove to a custody court that he could make a suitable mother.

"No, I didn't ever see Dorothy as an effeminate problem," Hoffman says. "And anyway, I wasn't trying to be a sexpot. Remember, I've played a vegetable and I've also played Ratso Rizzo, and this was just another acting problem, really. The thing is, we have no idea what it's like to be the opposite sex. Men and women, I found, relate totally differently. It's like if you're both the same sex, you're a person, but if you're not, you're poison."

"Oh, that's Dustin," says Streep with a tolerant smile. "I've heard all about it. What Dustin really wants to do, you know, is give birth. But he's still glad he's got a penis .

Gliding into a hotel room the morning after the floodlit New York premiere of "Sophie's Choice," Meryl Streep reveals that the camera does not, in fact, flatter her. She clasps her ankle-length black leather skirt and drifts onto a couch.

The Polish accent of Sophie is gone from her voice now -- as is the desperate need to be accepted, the foreshadowings of doom and the pathetic vulnerability she brought to that part. Behind her eyes is a quick intelligence that warns strangers not to pry.

Yes, she says, she is a feminist. Yes, as an actor she tries to make the part fit her point of view when possible. Yes, she can define feminism.

"It means the usual things. Let her get a mortgage, a loan, let her be an equal. It shouldn't be a dirty word. I tried to get some of that into "Kramer," and the director, Bob Benton, welcomed it. My character was in trouble, and she was smart enough to know it. She thought her husband was stable, and he could take the child while she went away and fixed herself. Then when she came back, when she had gotten herself together, she was still his mother. But then I went to the Academy Awards and Rona Barrett came up and accused me of having made an antifeminist movie."

She shrugs, sighs a characteristic and ambiguous sigh. She doesn't feel like bearing any banners, but she also isn't about to say things are grand for women in her trade.

"It's a weird time for women in the movies. They're usually victims, with no real sense of themselves. That's why I liked Sophie so much. Otherwise, if you're the heroine, you're either Joan of Arc or a clown."

Her idea of a heroine is neither, and she lets her ideas be known. The coffee shop scene in "Kramer," for example, was delayed while Streep argued that her decision to seek custody of her son should be revealed at the end of the scene, not at the beginning, as written. She succeeded in getting the scene changed, while Hoffman fumed.

"I finally yelled at her," Hoffman says, 'Meryl, why don't you stop carrying the flag for feminism and just act the scene!' " She got furious. That's the scene where I throw the glass of wine against the wall and it shatters. That wasn't in the script, I just threw it at her. Then she got furious again. 'I've got pieces of glass in my hair!' and so on."

Movies remain a world of men, for the most part -- male directors, male stars, men working the cameras, the sound equipment, the lights.

"I don't understand men," Streep says. "But I do remember wanting to be a boy for a while. The benefits weren't lost on me, I mean. That's the appeal of Meg in 'Little Women.' Anyway, I've never felt real feminine. I never understood what that was. Secretly, in different parts I have played, I have used a man as my model."

Hoffman studies for his roles, picking over humanity at large like a connoisseur at a flea market. To get Ratso Rizzo's characteristic limp, he says he scoured the streets of New York until he found a fellow with a particular rolling swagger, and copied him. For "Kramer," he looked to his own divorce, and for "Tootsie" to the memory of his late mother, and to Polly Holliday, who played television's "Flo" and is a friend.

He is now married to Lisa Gottsegen. They have a young son, Jake, and Hoffman has two teen-aged daughters from his previous marriage. The study of women, he says, came naturally -- but the discoveries of playing a woman were the surprise.

"Even though my perspective has changed, I'm still who I am," he said. "As far as women are concerned, I've always been interested in them. I'm totally dependent on them--they scare me, they comfort me, and they possess my imagination. I keep telling myself I'll grow out of it, you know? Hey, I'm 45, pretty soon I'll be 50. But then a friend gave me a series of Picasso prints. They're all about women, and he was 90 when he did them.

"The funny thing is, I was never able to be aggressive with women before I was successful. I had studied music and I played the piano, so I'd be the one at the party playing sensitive music like somebody in a James Dean movie. Some girl would come over and sit next to me, usually a shy person, and she'd say, 'You're nice.' I was a counterpuncher. I wasn't aggressive.

"This did change after success. Then, for the first time, I was told by the female public that I was attractive. But I think it's interchangeable. If it's Redford or De Niro, they'd say the same thing. It doesn't have much to do with me.

Streep's next performance will be as the title character of "Silkwood," a film about Karen Silkwood, the nuclear technician who was killed when her car ran off the road in 1974 en route to a meeting with a reporter. She was to have given the reporter information about alleged fraud and safety violations at a plutonium plant near Oklahoma City.

Silkwood, Streep says, of all the characters she's played, has the most in common with herself.

"She's very different. I think Texas women, in general, are different. I met her father. They'd had high hopes for Karen. But then she skipped out on a science scholarship. She left her husband. This was the early 1970s, and she lived as a swinging single in Oklahoma City. Believe me, she was a real tough cookie to reconcile with Joan of Arc. She was hated. She smoked dope in the plant. But the thing about Silkwood was, she caused a lot of trouble -- and she was right."

For a woman playing such a woman, Streep says, there are many challenges -- presumably audiences must find her sympathetic even if she's imperfect. Male heroes don't face the same handicaps. "The fact is that if Karen Silkwood had been a guy, there'd be no problem at all. She'd be played by Jack Nicholson."

Streep, who is 33, says she doesn't see herself as anybody's role model. "Really, I don't think anybody wants to be Meryl Streep. I think, at first, kids want to be Wonder Woman, and after that they don't have any role models at all.

"Everybody's confused. As a woman you still hide your light under a bushel. You don't want to frighten anybody off. If I'm not confused it's because I don't have any money problems at the moment. I'll have them when I'm 40, and suddenly I'm a character actress. All I know is my friend Colleen Dewhurst is brilliant, and she has a hard time working. There just aren't things for her to do, they aren't writing the parts and they never were.

"In the movies of the 1930s and '40s, we had all those wisecracking, headstrong women, but it wasn't the same. They weren't really competing. They weren't going to become lawyers and corporation heads, they weren't going to be making more money than the men."

The roles of men and women don't reverse easily, she believes. Great insights aren't to be realistically expected. A case in point is the inevitable, not-too-smart sex object--which only works one way.

"There'll always be a place for bimbos," Streep said. "But it only works with women. Bo Derek had a bimbo in 'Tarzan,' and it didn't work. Nobody believed it. The closest you get to something like that with men is Burt Reynolds. But he's not a bimbo -- he's a smart, fast-talking, charming adventurer."

Dustin Hoffman says he has realized, "There are a lot of women out there getting a raw deal," and that he wants to "feminize" more in the future as he did for "Tootsie." He tried it in "Kramer," and one of the reasons "I didn't get off on that movie more" was that some of it didn't make the final cut. "I saw Kramer as a lousy father who became a good father and who learned to be a mother. I improvised a courtroom scene where, when they claimed the boy should be with his mother, I shouted, 'What do you mean, I am his mother!' But then they didn't use it, and as an actor you can't do anything about that."

He says he's as puzzled as anyone else about the definition of a man and a woman in the 1980s.

"But I'll tell you this. I have a friend in Connecticut, he's a big, burly sculptor, and when I saw him last he was wearing an apron. He tends to his son, he does some housecleaning, he's out there feeding the ducks and geese. His work is very sensual, it's all about the sensuality of nature. He said to me, 'I never walk past a tree without seeing a vagina.'

"And I realized, when I came back from visiting him last, that this is the most masculine and virile man I'd ever known."

Streep, who happens to be married to a sculptor--they learned last week they are expecting a second child -- hears Hoffman's observation reported without comment, but with an inscrutable smile.

In good acting, they tell us, the sparks have to fly. And in the war between the sexes, it would appear, the time for mutual disarmament is not yet at hand.