The letter was from a youngster who had watched Mariette Hartley film some scenes for "Silence of the Heart," a TV movie in which she plays the mother of a teen- ager who commits suicide.

"I thought of killing myself once," read the letter. "But I think I saw in your performance how my mother would have felt had I succeeded. Thanks."

The message brought satisfaction to the actress and suggested that "Silence of the Heart" (Tuesday at 9 on CBS) might fulfill the wishful tagline that inevitably is attached to such a movie. With "Adam," the tag read, "If one missing child can be found, the show will be worth it." With "Silence," it says: "If one teen-ager can be stopped from taking his own life . . . ".

And for Mariette Hartley there is another footnote to "Silence of the Heart": If a final payment can be made on a 20-year debt to the past . . .

D "It was time," said Hartley, for the filming of "Silence of the Heart." "There've been 14 cases (of teen-age suicide) in Dallas, five in L.A. There are three suicide stories coming out on TV this season . . . This one deals with a regular family. No one's a drug addict or anything. People can look at it and say, 'That could be us.'>"

Hartley has enjoyed being part of the recent television trend toward movies dealing with hard-hitting social issues. Her earlier contribution was "M.A.D.D.: The Candy Lightner Story," which dealt with the campaign for stiffer penalties for drunk-driving.

"I'm glad such movies are being done," said Hartley. "It's the best use of TV." Feature movies dealing with topics like teen-age death are not going to draw crowds at the theaters, she said, with outstanding exceptions such as "Ordinary People." But such shows will be seen by millions on television. "TV," she said, "has a captive audience."

And there was added incentive for her to do "Silence." "It was such an inspiring piece for me to do," she said, "from my own experience."

In 1962, Hartley, at 22, appeared headed for stardom in a terrific rush. She was a New York-born actress who had stormed through childhood in prodigious style -- college admittance at 16 and extensive stage work (some of it under John Houseman, some under Joseph Papp) by her late teens. And she had completed "Ride the High Country," a Sam Peckinpah western with Randolph Scott and Joel McCrea. The film is now regarded as something of a classic of the genre and Hartley received good notices. "But movie executives fell asleep on it," she recalled. "I felt like I'd been shot down."

There was further promise, though, in a movie called "Come Fly With Me" to be filmed with Karl Malden and Pamela Tiffin in Europe. Hartley was to be in it too until she ran afoul of a studio insurance doctor whose approval was needed for her to make the trip. In a strange sequence of events, he misdiagnosed her as having hepatitis and subjected her to a bizarre blood treatment. Lois Nettleton got to do the part. A few weeks later, the doctor killed himself.

Then fell the blow that was to leave a lasting impression. Hartley's father -- an advertising executive who had passed on to his daughter his wish for her stardom, and the burden of fulfilling it -- committed suicide.

She did not work for more than a year. In the aftermath of her father's death, "it was a struggle to find myself. I was on the verge of a nervous breakdown. There were too many defeats. I finally admitted defeat and went into therapy."

Then came her role of Leslie Nielsen's wife in the "Peyton Place" TV series. This, she notes, was during her "frigid wife-frigid doctor period," which soon became her "plain- passive-sister-always-loses-the-guy-to-my-prettier-sister period." There were featured roles in TV series -- "Gunsmoke" and "Ben Casey" -- and a part in "The Hero," a satirical series that failed ("We were 10 years ahead of our time.")

By the late '60s, Hartley was asking herself a series of questions: Did she really want to act? Was she merely fulfilling others' fantasies? Could she do anything else well?

To find out, she left the profession and became a saleswoman in a series of boutiques. Success came in sales, and so did the desire to return to acting. She returned to the theater. Among the plays that earned her good reviews in Los Angeles was "Chemin de Fer," a French farce with Hume Cronyn and Jessica Tandy. She also did some guest spots on television and started to dabble in commercials.

Dabble? She did about 100 of them. And that was before Polaroid came along. It was during a commercial tryout that she met Patrick Francois Boyriven, a French director, whom she married in 1976. They have a son and daughter.

Hartley's guest-star appearances included episodes of "Columbo," "Police Woman," "Delvecchio," "Streets of San Francisco" and "Barnaby Jones."

Her most important guest shot was an offer she nearly refused. "My agent called and said I was going to do 'The Incredible Hulk,'>" she recalled. "I was breast-feeding at the time." The Hulk, Bill Bixby, called her and urged her to come to Hawaii to film the segment. "If the baby cries," he said, "we cut." She accepted. Her role in that one episode -- portraying a dying woman -- won her an Emmy. Last season she teamed up again with Bixby for CBS series "Goodnight, Beantown."

A commercial proved to be a step up from the Emmy. "Before the Polaroid commercials, my image was that of a solid actress, a theater actress who could do anything," she recalled. "But the Polaroid commercials were high comedy . . . Through them I was finally noticed as a commedienne.

"A week before the Polaroid offer came up, I had decided not to do a commercial for scale (pay) again. My agent said, 'Do this one.' We didn't know at the time that it would turn into a campaign." It did indeed. She estimates that she cranked out 350 commercials, ranging from 10-second to full-minute spots. And in the process, she and James Garner became so closely identified as a couple that they spent years denying they were married to each other.

Scale at the time, she said, was $175 a day while Garner was pulling down $33,000 a year from Polaroid. To his credit, she said, he helped her get more money. "Not as much as he was making," she said with a smile, "but more."

The Polaroid spots made Hartley's a household face just about the time she had given up on fame. "I had married, had a child. I was content to stay at home," she recalled. "I had let go of the fantasy of stardom."

About that time, Fred Silverman, president of NBC Entertainment, decided to seize upon her stardom. He gave her a shot at co-hosting the "Today" show while Jane Pauley was away. She worked for three weeks with Tom Brokaw. "Freddy dropped me there and headed for Hawaii," she said. "I said, 'Freddy, you can't do that to me. You need to tell people what I'm here for.'>"

While Silverman was lying on the beach, Hartley recalls trying single-handed to unsnarl the tangle of mixed loyalties among the program's staff and trying to hit her stride in an unfamiliar setting.

"The first two weeks were rough, but I got good at it by the third," said Hartley. "I was honest. If I couldn't find the red light, I said so. I never approached it as a news program. I thought of it as an entertainment program. They're there to wake you up and start your day off."

In the end, she said, the staff resistance was too much. "They celebrated my birthday," she recalled. "But I told them, 'I don't know what I've done to all of you.'" She pondered the personal conflicts: "It was a snake pit."

Hartley credits her husband with helping her into and through the "Today" stint. "He took care of both kids. I couldn't have done it without him," she said. "He helped me with research. He talked me into doing it in the first place. 'You've got to try,' he said. 'You'll learn a lot.'>"

Now Hartley's career has evolved into television that attempts to be relevant, though she harbors a dream to do the absurd.

"I'm finally doing some responsible things," she said, referring to "M.A.D.D" and "Silence." "But I'd like to do a comedy. So much of my career has been a matter of taking what you have to . . . I've begun to trust my talent the last four years. I've been in front of the camera the last 20 years, and it's just become a friend."

In "Silence of the Heart," the camera is a friend and the script is, perhaps, a psychoanalyst, beckoning her to the past. Was making this movie therapeutic?

"I think it was," she said. "I had a dream about my father during the filming. I think I finally made peace with him."