Neil Simon -- the playwright who has tickled America's funny bone more consistently in the past 20 years than anyone else -- sits on a plush couch 35 floors above Park Avenue and contemplates his fate.

"Here I am at the highest peak in my career," he says, "and I'm at the lowest point in my personal life. And they're both going on simultaneously. There's some odd joke in that."

But for the moment, damned if he can find it.

First, the silver lining. The King Midas of humor, Simon has converted odd couples, red hot lovers, sunshine boys and husbands who go barefoot in the park into an emperor's fortune. His newest film, "Max Dugan Returns," is shaping up as a box-office smash (its first weekend out, it grossed more than $2,700,000 in 790 movie houses across the country). Meanwhile, his latest play, "Brighton Beach Memoirs," has won him his best reviews from the New York critics in years and is settling in for a long and lucrative Broadway run. What's more, the critics, who have sometimes begrudged the playwright his ability to coin more funny lines per minute than seems humanly possible, have now decided that he has a very warm heart. Neil Simon, who arrived a long time ago, has in a manner of speaking, well, arrived!

Now the cloud. After 10 years of marriage, Simon is splitting from Marsha Mason, his actress wife and the costar Simon makes no bones about being a family man, needing that warmth, the support, the routine. Between his two wives, he's had 28 years of marriage behind him and can't conceive of life in any other terms.

It took him only four months, after the traumatic death from cancer of his first wife, Joan, to fall in love with Mason and marry her. Their whirlwind courtship formed the basis of "Chapter Two," his most overtly autobiographical play until "Brighton Beach." When Mason wanted to go West to get a film career going, Simon willingly followed. He even wrote most of her films (among them: "The Goodbye Girl," "Only When I Laugh," "The Cheap Detective").

"I've never felt alone writing in a room," he says, "because I've always had a family to go to at the end of the day. Without that family, writing is the loneliest life in the world. I don't want to go into late middle age saying, 'I've got to have another play, I've got to have another play.' I don't need another play now. I need a life. I'm 55. I have a lot of years left. I don't want to waste them over a typewriter. I'd rather find my sustenance somewhere else. The joy of writing has stopped, because the joy in my life has stopped."

He pauses--a vaguely overweight, tanned, middle-aged man with thinning black hair, owlish glasses and expensive tennis shoes. This morning he awoke with a firm resolve in mind: He's put all of his projects on hold for a minimum of a year. Maybe two, before tackling the film version of "Brighton Beach Memoirs." He's going back to his Bel Air home, because that's where he feels his roots are--not the Park Avenue apartment he views primarily as a pied-a -terre, even though its dimensions render the term slightly ludicrous. Perhaps he'll do a little traveling. But mostly, he wants to think this mid-life crisis through.

"Maybe this split is the best thing that could have happened to me," he says, wanting to believe it.

There's more cloud. He's spent most of the morning on the phone, learning that he's lost "a great deal of money" on some bad investments. The grim details have furrowed his brow. But even that he's trying to put behind him. "It doesn't mean that much," he tells himself, "even though it means about eight years of work in terms of money gone down the drain. But I would rather have my life together than think about that money. I've got enough. If I watch it, take it easy, there's no need to worry. I'm never going to be broke. The royalties will come in for the next 54 years. I want to be able to leave something to my two children, but not so much that it's going to screw up their lives. Mostly, I want to get over this period of being suspended."

If Simon feels he can abandon the theater for a while, it's because "Brighton Beach" represents a personal apotheosis, not simply another fat, commercial hit. Some critics have even gone so far as to say that it is his "Ah, Wilderness!," an observation that puts him in the company, if only for comparative purposes, of Eugene O'Neill. For the first time in his career, Simon has begun to think of the legacy he will leave behind. "I feel that if I had died without writing my 'Brighton Beach Memoirs,' well, I would have had a nice legacy as a writer of light comedies," he explains. "But it would have been incomplete. This play has satisfied me so much that if it were all over right now and I never wrote another play, I'd be quite content."

In it, Simon is writing for the first time about his youth, growing up poor and Jewish during the Depression. He's writing about his mother and the way she would pack him off to the corner grocery 16 times a day for a stick of butter or a quart of milk--never a pound of butter or a gallon of milk, because who knew what the following morning would bring and there was no sense in wasting money on food you might never consume. He's writing about her bark ("Stop that yelling. I have a cake in the oven") that was far worse than her bite. And her stern bigotry that was really just her manner of putting her imperiled family first.

"She left such a deep mark on my personal life," Simon reflects, "that I have both gained and suffered from having this bizarre, close relationship. I say bizarre, because it seemed so at the time. And yet she nurtured me so much that women and a domestic life are terribly important to me, despite the fact that I lived in broken homes all through my childhood. My mother was the one who stood by me and I knew that whatever I did, it was wonderful with her. When you grow up being loved like that, you gain a great deal of security and confidence in yourself. On the other hand, you grow up wanting and needing so much that when it's not there, the loss is enormous. For all those opening nights, all the plays and movies I did over the years, I never had an ulcer until my mother died. That's when the acid started flowing."

He's writing about his father, or, more accurately, the principled father he always wanted, since his real father was constantly disappearing for months, while the remaining Simons, with mother at the helm, were shunted from relative to relative, trying to keep up a semblance of home life.

He's writing about his big brother, Danny, who told him, prophetically, "You're going to be the best comedy writer in America." Simon still marvels at that. "I was 14, and maybe once in a while, I'd say something amusing around the house. But he picked up on it. He noticed. I idealized him. I realize now that he bore the brunt of the terror that came down to the children. I was spared a lot by his being in the way. He paved the path for my whole career. We used to go out for writing jobs on radio and early TV shows and I never could have lasted without him. Although I did most of the writing, Danny was the spokesman. He got us through doors, negotiated things, kept the team going and gave me encouragement and energy."

Mostly, though, Simon is writing about himself in the guise of 15-year-old Eugene, who serves as the play's narrator and whose cockeyed slant on the family's tribulations keeps the play in comic perspective. As played by Matthew Broderick, Eugene is the most immediately appealing character in "Brighton Beach Memoirs." At curtain call, audiences cheer.

"He's a lot cuter than I was," admits Simon. "But that's the way I remember it. I decided that Eugene would be the only character in the play who's funny. Take him out and you have a straight play. But his persepctive on life is my perspective. It's not that I try to make things funny. It just happens.

"There've been a lot of tears in my life in the last 10 years. Before that it was all joy. The career was going upwards, the kids growing up, it was terrific. Then my first wife died. That was so devastating to me I literally thought I wouldn't live through it. Then your children move away. Your parents die. I had to adjust to change. But I still try to maintain the humor. That's what makes whatever I do unique and different. In the deepest, darkest hours of my despair, I say something funny as a life-saving device, I guess. Whether I do it consciously or unconsciously, I'm putting things in perspective. Really, it's the luckiest thing about me.

"I think it's what attracts people to my work. It says to them, 'Look, he can laugh at some of the same problems we're going through.' Some of them, not all of them. We're certainly not going to laugh at death. But I think audiences feel if my characters can get through problems without that deadly seriousness, well, maybe they can, too."

Even as Simon is trying earnestly to plumb his current pain, the old life-saving mechanism clicks on. "I began to find out that life is filled with disillusions early," he says. "As a kid, all I wanted was enough money to go to the Polo Grounds any time I wanted to. And when I did, the Giants picked up and moved to San Francisco."

Lighthearted as it is, the fantasy that is "Max Dugan Returns" is also rooted in Simon's remembrances of his fractured upbringing. The story is contemporary Cinderella. An attractive widow (Mason) in Venice, Calif., is struggling with heroic good spirits to raise her teen-age son (Broderick again) on a paltry teacher's salary. But it's tough going. The kitchen appliances are on their last legs and her ramshackle Volvo is stolen out from under her nose. Then her charming wastrel of a father, Max Dugan, shows up. He had abandoned her when she was 9, has served time in the clinker and informs her he's got six months to live. He also has $687,000 in neatly packaged bills in an attache case and promptly sets out to buy her and his grandson every gift their hearts could desire.

"I was afraid to write it at first," Simon acknowledges, "because it came right after 'I Ought to Be in Pictures,' which was also about the reunion of a father and his daughter. I think it was me still trying to resolve the relationship with my own father. Growing up, we always seemed to be going through reunions. Partings and reunions. The partings were painful and the reunions were terrific.

"But the story just popped into my mind and it seemed so timely. Then I got nervous about the movie. It's such a simple story about simple people. I think audiences are looking for warmth, but someone out there--the producers, the critics--are pushing away from it. It seems to me the grimmer the piece, the more the critics like it. But we all need our fantasies. They're what get us through. We deal with reality enough."

The current round of huzzahs notwithstanding, Simon nurtures a certain skepticism toward the critics. "They accepted the lesser plays much more vigorously than the latter plays, which were better," he says. "Certainly, 'Prisoner of Second Avenue' is a better play than 'Barefoot in the Park,' but the hats went into the air with 'Barefoot.' They said, 'Here's a new, young playwright who's going to give us a lot of wonderful light comedies.' But you give them enough light comedies and they say, 'We want something deeper.' So you go to something deeper and they say, 'Where are the light comedies we liked so much?' You realize that they want you to write what they want to see. And they all want to see something different!"

In Simon's estimation, the one that counts with him, "Brighton Beach Memoirs," "is about as high as I've gone so far." But it's also been consuming. He used to write a play, then go off on vacation and enjoy the fruits of his labor. "Something's been driving me these last few years. I don't know what. If I did, I'd have the answer to my problems. In one sense, I'm glad because I came up with my 'Brighton Beach Memoirs.' But I'm also sorry, because I've spent so much of myself."

His tone is without self-pity. There's even a kind of professorial sobriety about him as he sorts through the glory and the wreckage of his life. "I hear I'm the most eligible bachelor in town right now," he say a bit ruefully. "People tell me, 'You can have any girl you want.' But any girl you want isn't the right girl. It's like writing any play you want. That doesn't mean anything unless it has quality. It was almost easier for me to fall in love quickly with Marsha, the way I did just four months after Joan's death, because Joan was gone. And when someone dies, you know you have to let go. But I can't let go of Marsha yet, because she's still there. And she hasn't let go completely either.

"We've parted. We're getting a divorce. But I don't consider that the end. There's no law that says you can't get remarried again. We still talk to one another on the phone every day. From the conversations you'd think it was Romeo and Juliet. But I want to change my life and that takes more courage than anything else in the world. I've got to make the move. Go out and seek it, whatever it is, and not wait for the doorbell to ring and someone to say, 'Hi, I'm the beautiful girl next door.' "

So Simon is heading back to California. No slow fade-out and swelling music this time. Where the sunset usually is, there's a big question mark.

Seemingly, he's had it all--acclaim, success, wealth beyond most mortals' daydreams--while the rest of the world fights merely to keep up with the mortgage payments. He must be a nice guy. Because when he says he just wants to find out what his life is all about, you suddenly find yourself wishing him--this, the most successful playwright of his generation--good luck and Godspeed.