Just one year ago, when a research firm asked teenagers who their favorite musical performer was, the bulk of them said Whitney Houston.

When the firm posed the same question to teenagers last month, the response was Whitney who? Houston wasn't even rated in the top 10. This time, pop star George Michael topped the list.Four years ago, Michael Jackson received the highest popularity rating the research firm has ever recorded for a performer. But last month, Jackson barely squeaked into the list of the top 10-rated performers - as rated by teenagers.

"Teenagers are into anything that is hot," said Peter Zollo, president of Teenage Research Unlimited, the Northbrook, Ill., research company that surveys 2,000 teen-agers (12 to 19 years old) every six months. "As soon as someone isn't hot - maybe because they don't have a new album out or they're not on tour - they're all but forgotten."

Indeed, even George Michael - who has been popular among teens for only about a year - is already starting to cool off, said Zollo. The next survey, planned for this fall, he said, will likely show that Michael - who recently signed a multimillion-dollar contract to promote Coke - has slipped several notches in popularity.

So who is paying researchers many thousands of dollars to come up with information like this? Not just record companies, that's for sure. More and more, it is advertisers - especially soft drink and fast-food makers - who are ferociously vying for their cut of the lucrative teen market. Teens spent more than $79 billion last year, up more than $5 billion from 1987.

Although the nation's teenage population has decreased slightly - down about 1 percent last year from the year before to 28 million - the spending power of teenagers continues to rise. With allowances and the money that parents give them to buy items like clothing, the typical teenager spent $61.50 per week last year, said Zollo.

But executives say the real reason that Pepsi and Coke are spending millions of dollars trying to lure teens is this: Soft drinks are about the only food and drink category where teenagers show brand loyalty.

Yet Pepsi and Coke are finding it increasingly difficult to appeal to teens as a single market. "A sweet 16-year-old will probably not like the same music that her 18-year-old sister likes," said Steven Levitt, president of Marketing Evaluations Inc., a Port Washington, N.Y., research firm.

"In order to interest teenagers," pointed out Zollo of Teenage Research Unlimited, "you not only need to find singers that separate them from their parents but also from their older brothers and sisters."

That is why Pepsi, for example, not only signed Madonna but also singers such as Robert Palmer. And beyond rock rage George Michael, Coke says it is searching for other hot singing stars to tout its brands.

Part of the problem is that no singer has universal appeal among teenagers these days.

For their part, the big soft drink makers say that's fine with them. "The star isn't the star. The product is the star," said Georgia Camp, a Coke spokeswoman. Concurs Tod MacKenzie, a Pepsi spokesman: "As big as Madonna's and Michael Jackson's names are, Pepsi's is bigger."

This has recently forced some advertisers to seek avenues beyond rock music to entice teens. Pepsi's Slice brand, for example, recently began a promotional tie-in with the popular Nintendo video game. And Coke has invested more heavily into teenage event marketing, such as Diet Coke's promotional sponsorship of various activities in Daytona Beach, Fla., during spring break last month.

Still, the soft-drink makers continue to chase after the teen heart-throbs of rock. Executives from both Coke and Pepsi indicated that they could both be interested in Debbie Gibson, an 18-year-old singer and song writer whose recent concert tour was co-sponsored by Revlon.

Many young teens - especially girls - are big fans of Debbie Gibson. In fact, Gibson rated second only to George Michael in the recent Teenage Research Unlimited survey of teens. "Not every teenager likes Debbie Gibson," said Jay Coleman, president of the Rockbill, a New York promotional company that links up advertisers with performers. "But she has enough critical mass."

And even though singer Tiffany is very popular with teenagers right now, she is probably not a good bet for advertisers, executives say. "She mostly sings other people's songs, so in the long run, teenagers might feel distanced from her," said Zollo. Likewise, popular heavy metal bands - such as Guns & Roses or Quiet Riot - are unlikely to soon be singing for Coca-Cola. "It's too difficult," said Rockbill's Coleman, "for corporations to attach themselves to rock groups that seem rebellious."

Recently, even Campbell Soup Co. got into the rock action. After research showed that teenagers were gobbling up Campbell Chunky Style soups, Campbell began to advertise the brand to teens. "If you can convince a kid to be brand-loyal at 18," explained Anthony J. Adams, vice president of marketing research at Campbell, "that's maybe 50 years of product sales. That one customer can be worth $150 a year."

To interest teens, Campbell went directly to the MTV network with the Chunky soup campaign. Unlike most Campbell ads with those lethargic tunes - like "Mmmmm, Mmmmm, Good" or "Soup Is Good Food" - this commercial featured a teen heating up Chunky soup to the rocking oldie "Soul Man."