IT WAS an idea conceived late at night in a Los Gatos, Calif., bar, and nine months later Gary Dahl's Pet Rock made him a near millionaire.

"The rush (Christmas, 1975 - remember?) lasted about three months," says Dahl. "We were up to our ears in debt - about $200,000 - before anything started coming in. But once it did, it was an avalanche."

Such an avalanche that Dahl remembers, for example, writing a check for $39,000 to a West Coast representative for one month's work. An East Coast marketing man sighs, "I'd like to see something like the Pet Rock again. You should see my Jaguar."

So far as Dahl is concerned, "The rock created and destroyed the novelty gift market all at the same time. We (Rock Bottom Production) came out with a sand-breeding kit the next year that was a real bomb. But the rock helped me fulfill my real dream - opening my own pub."

Dahl hasn't forgotten, however, the days he and his wife were assembling the $5 joke rocks, eye-catchingly packaged with "training manuals."

"We were working in our home, a small cabin in the Santa Cruz mountains. In 1 1/2 months we needed 300 employees.

"Sure, we live differently today," said Dahl by phone from Los Gatos. "We drive Mercedes instead of Hondas; instead of a Franklin stove we've got real heat, and our swimming pool is bigger than that whole cabin."

But lest anyone else think they've got an easy, million-dollar idea, Dahl, 40, stresses that his background was in marketing and advertisng. A "very brief student" at Washington State, he was working as a $25,000-a-year creative director for an advertising agency when struck by the rock idea.

"I had an advantage over the average backyard inventor," whom he frankly wishes would leave him alone. "People somehow think I owe them something because I made it. I've had thousands of letters and phone calls from people requesting help. Any day I can arrive here at my saloon and find people waiting to talk about their idea.I wind up being rude, which I don't like to be. But my advice to people wanting to get into the novel gift business now is to stay out of it."

Dahl spent a quarter of a million dollars to refurbish - complete with hand-rubbed mahogany and stained-glass windows - his Carry Nation saloon in Los Gatos, about 50 miles south of San Francisco. "It's a damned nice place, like a well-lighted and elegant old English pub, where little old ladies can come and sip their strawberry daiquiris and watch the world go by." Like his Pet Rock, his pet pub seems to be paying off. Gross for its first year: $400,000.

"And now," he says, "if people would just forget I did the Pet Rock, I'd be happy."