Kirk Cameron is enjoying success as a big screen heartthrob, and his popularity leaves him surprised.

"Heartthrob" is an old-fashioned and perfectly suitable definition for an actor who captivates millions of females, especially the young.The first movie heartthrob might have been Douglas Fairbanks or Rudolph Valentino. Cameron's fans believe he fits the bill.

At 18, slender of build and boyish of smile, Cameron is indisputably headed to major stardom.

His recognition is growing fast, thanks in part to his weekly "Growing Pains" television series and to last year's co-starring role in the Dudley Moore comedy "Like Father Like Son."

Now Cameron has a new movie, "Listen to Me," in which he plays a college student who's a hot-shot member of the debating team and is involved in a close relationship with two of his classmates.

Despite his huge following, Cameron remains a friendly, self-conscious youngster who seems astonished at becoming a heartthrob for the pre- and post-adolescent set.

Seated on a divan in his publicist's office recently, Cameron held out his hands, palms up, shrugged his shoulders and shook his head when asked how he became a screen idol.

"All this excitement got started about four years ago," he said. "I guess it's because of my role in `Growing Pains.' I get about 10,000 (fan) letters a week.

"Most of them want autographed pictures. Some want me to write back or stop by their houses on Friday night. Some ask for dates or advice.

"Once in a while I call if I think I can help. I'm very involved with the Just Say No (anti-drug) organization. And a lot of kids with drug problems can use help.

"My popularity surprises me because I never really had my heart set on becoming an actor. I never said, `Mom, I gotta do commercials.' I just sort of fell into it when I tried to pick up a little work for money."

"I never expected this much focus on me."

Cameron has a winning way about him and appears younger than his age. He has a sort of wholesomeness not seen in the likes of a Rob Lowe or Sean Penn.

Cameron sees himself as a role model for youngsters.

"I wouldn't be cast for a picture like `Risky Business,"' he said. "Sure, that movie was fun and everyone enjoyed it. (But) I'm selective about my work because I won't compromise my personal values for a role just to be successful.

"I really hope people don't think that kids my age can hardly wait for their parents leave town before their lives begin.

"People see me as sort of a substantial citizen. It's not something I'm trying to be, it's just what I happen to represent on the screen. That comes from me wanting to communicate what works best for me in my life.

"I try to be true to who I am and what I believe. Maybe other people will find that worthwhile."