Saturday night the "Social Security" ensemble, headed by Marlo Thomas and Ron Silver, director Mike Nichols, writer Andrew Bergman, coproducers Bernie Jacobs and Gerald Schoenfeld (of The Shubert Organization) and David Geffen, seemed on the tired and cautious side of happy. After the opening at the National Theatre, a gathering of about 100 guests assembled at the Bristol Hotel for a cast party hosted by Nichols.

The conversation -- less seltzered with one-liners than the play's dialogue -- was more akin to the pastel and low-ceilinged decor of the Bristol's downstairs lounge. Cast members Ron Silver and Olympia Dukakis leaned against the bar and talked quietly. Thomas and her tweed-jacketed husband Phil Donahue arrived without fanfare, settled down (first at separate tables and later at the same table) and turned out mellow chatter.

With the celebration of the birth of another Broadway-bound play came the gentle subsiding of opening-night adrenalin and anxiety. Donahue relaxed with a Heineken; Thomas sipped white wine.

Had Donahue given Thomas any advice? "No, no . . . For me to give Marlo advice would be like trying to tell Dr. DeBakey how to perform surgery."

"With Mike Nichols, how can you lose? And Bergman doesn't miss anything," said Thomas, of the "wonderfully funny" script. "A lot of plays are easy not to do," she said, but "Social Security" was not one of them.

Actor Stefan Schnabel, white-haired and veteran, acknowledged he was "not a laugher." But when reading "Social Security" back in December, Schnabel said, "tears were rolling down my cheeks."

With confidence apportioned to the star, anxiety remained for the star's husband. Less like a cool surgeon and more like a husband pacing the waiting-room floor, Donahue had stood in the back of the theater throughout the performance, feeling that his "clinical, analytic" mode didn't belong in the fourth row.

"Marlo worries about me when I am out there doing something special . . . I worry about her." Actually, he went from "understandably nervous" to "nervous wreck" in his self-appointed position as "a cheerleader of the star."

Minutes before, while discussing the themes and the humor of the play, Thomas had said, "The only people that can drive you crazy are the people you love."

In what appeared to be a macrobiotic violation of Thomas' diet, Donahue offered his wife a dish of strawberries and whipped cream.

When asked if her macrobiotic chef, whom she brought down from New York, had watched over the party's menu, she said: "I'm not that strict." The buffet dinner included wild rice salad, carponata, sesame chicken strips with apricot-orange sauce and mushrooms stuffed with veal. As one of the play's characters had said earlier in the evening, "Dinner is dinner."