Marlo Thomas opens the door of her dressing room at the National Theatre and it's like a little sprite coming out of a bottle. Her tininess shrouded in a loose sweater (she's one of those people who fight to gain weight), she gleams with fashionable and well-cared-for polish.

Thomas is here starring in "Social Security," a new comedy that opens tonight. It is her first play in 10 years. The counters in her dressing room oare covered with flowers from family and friends, and there is a satchel full of macrobiotic goodies and a thermos of a low-caffeine drink made of tea twigs.

"I've been exploring eating better for the last couple of years," she says in That Voice, the sound of a coy frog. "When I was on 'That Girl' and everybody else in the studio was taking bennies, I used to say there's got to be another way to work real hard days. I used to eat Hershey bars -- that was my way of staying up. My nickname was Hersh."

She brought her cook with her from New York, where she lives together with her husband, talk show host Phil Donahue, and a bunch of exercise machines. Twice a week her personal trainer stops by to check her workouts. In short, she looks very good at -- well, one reference book makes her 48 but the more recent ones have her 42. Whatever she is, time has been kind.

Thomas' career has been fairly eclectic, stretching from early jobs in summer stock and bit parts on television to the five years of "That Girl" (starting in 1966), back to theater and on to television movies. Along the way she produced and starred in an award-winning children's series, "Free to Be . . . You and Me."

She has been a visible feminist, a member of two presidential commissions relating to women and children's issues, and an occasional witness before congressional committees. She is also a director of St. Jude's Children's Research Hospital, founded by her father, entertainer Danny Thomas.

Her last three television movie roles were characters based on real people -- a woman hounded by the police after a one-night stand with a suspected terrorist, a mother dealing with her son's homosexuality, and a psychotic who confronts the deaths of everyone close to her.

The roles have appealed "to the truth seeker in me," she says. "I mean, I'm on a journey to find out the truth, and sometimes by living out someone else's journey you get an insight into your own . . . You do grow as a person."

To get from the evanescently and eternally perky "That Girl" to her recent roles, Thomas took a four-year sabbatical from working to study with acting guru Lee Strasberg in New York, living off earnings from the TV series.

"I felt that working in my television series kind of hurt my creative development," she says. "You learn a lot of shortcuts when you have to work a 16-hour day, you know? . . . I felt I really needed to go back and work. I felt I needed to begin again in many ways. And at Strasberg's Actors Studio I was able to play Ibsen and Strindberg and Chekhov and other things I would never have been cast in. It was very scary and very exciting. For the first six months I wore navy blue and didn't move from my seat, I was so afraid of being called on."

Although during that time she did one television movie and worked on preparing other projects, for the most part her career was on hold. "In the early years of your career it's 'Look, Ma, I'm dancing.' Then it's 'What else can I do?' It's like everything else in life. 'Is this is all there is?' . . . There were some things I turned down that I saw other people get Academy Award nominations for and I thought 'Oh God!' But I still think that was their Academy Award nomination and this was what I needed to do."

Six years ago she and Donahue were married after living together for several years. She had not followed the pattern expected of her generation, marrying in her early twenties and producing children.

"For years people would say, 'Why don't you want to be married?' My mother used to say, in her wonderful way, 'You'll die alone!' And then my father would say, 'Well, you're living with him, why don't you marry him?' And then when I got married everybody said to me, 'Why did you get married?' Well, I just can't deal with everybody else's sketch for my life."

When she first moved in with Donahue, he was living in Winnetka, Ill., with his four sons, then ranging in age from 11 to 16. (His daughter lived with her mother). For Thomas, who had lived alone most of her adult life, it was a shock.

"It was just a disaster," she says. "Everything was a mess. Everything was broken. There wasn't a towel -- you couldn't even dry yourself. It was like going into this gigantic Army camp. I immediately set to work color-coding the towels -- 'The white ones are mine! Don't touch them!' . . .

"I had no place to go. We had a bedroom, and the children each had rooms. The family and living room were open. I used to read in the dining room because nobody went there. The dining room became my friend."

But, as might be expected, she found the role of stepmother "very expanding." The five children, now in or just out of college, come home to the Manhattan apartment for vacations. "It is an invasion. For someone like me, who's very orderly, to suddenly be surrounded by pizza boxes and dirty socks and wet towels . . . "

And yes, even she -- the woman whose cheerfulness seems as much a part of her as her distinctive voice -- explodes. "I say 'NOW, LOOK!' In pursuit of fairness, five guys and one girl is not fair. You have to have some kind of respect for the way I like things to look. I don't want to walk into a gymnasium every day.' And everybody scurries around and tries. I have to bend. The summer is just a nightmare sometimes."

But there are rewards. She feels a stepmother can offer a less hysterical, more objective approach to crises. Her stepsons did not resist her arrival on their family scene, and all the children are "very loving," which is one of her highest accolades. Her stepdaughter returned from a year in Paris, during which she had received numerous bundles from Thomas and Donahue and her own mother. When they asked her if she'd gotten them, Thomas remembers, "she said, 'Oh, yes, it was so wonderful, to know that all three of my parents loved me.' "

"Of course, I wept," says Thomas.

In "Social Security" she plays a happily married New York art dealer, a sort of older yuppie who has all the right everything. "It's about family and sex and money and sex and family," she explains cryptically, cautious about giving away the plot. The play is scheduled to open in New York after its run here, and she hopes it will run for at least the nine months she has signed to play in it.

"I love the life of the theater, the ritual of it . . . but it is kind of disruptive for me and my husband because he works in the morning. We haven't figured out yet when we're going to see each other . . . Right now my husband is being so wonderful I can barely recognize him. He's taken over all the wifely duties . . .

"Women have always been the enablers. They enable all the men, all the children, to get the most out of life that they can . . . And that is one of the reasons I didn't want to marry, because I didn't see myself as an enabler. I wanted to be the one to go to school and to work and have somebody else worry. Phil and I always joke about who's being the wife this week. One of us has to be the one to take care of all the details, answer the messages and drop off things . . . I probably do it a little more because I'm guiltier."

"Marriage has opened a lot of doors inside me," she muses later. "Some good, some bad. Some doors marked 'Jealousy,' some doors marked 'Space,' some doors marked 'Teammate.' And one very large door called 'Trust.' I don't mean trust that he won't fool around or I won't. That's a given or I don't understand marriage. I mean the kind of trust that what's dearest to your heart will be taken as a priority. Our mothers used to say, 'Whatever makes you happy, darling,' but they didn't really mean it. Here indeed is someone who means it. If it makes you happy, I will be inconvenienced, I will -- that terrible word -- make sacrifices for you. I will help you flower. To get married is to trust somebody with your head, that they won't foul you up with what their needs are."

Her "journey" may have been unconventional, she says, but it has been all hers. "I remember when I was first starting in this business everyone was comparing me to my father. And it was a very hard time for me. And I said to my father, who I love and am very close to, that I hated it and it was very hard to be compared to someone else. And he said, 'I've raised you to be a thoroughbred, and thoroughbreds run their own races. They don't look at the other horses; they put on the blinders and run.' Later I was doing "Gigi" in summer stock and this big white box arrived in my dressing room. I opened it up and it was an old pair of horse blinders. And it said 'Run your own race.' And that's what I really think. You have to run your own race. You only have one; it might as well be your own."