"Wanted: People aged 18 to 29 to take recreational drugs as part of a scientific study. Earn up to $400 in four afternoons."

That recent newspaper advertisement is on the up and up. It's part of the quiet battle waged by drug companies and the federal government to find ways to curb recreational use of prescription drugs.Scientists hire the volunteers to learn what makes some drugs so pleasant that they are likely to be abused. They hope to use that knowledge to develop drugsthat are as effective but not pleasurable enough for illicit use.

Drug companies use the studies to decide whether to market a new drug, and inwhat dosages. Federal agencies use the data to decide whether to approve a drug for sale or impose certain restrictions on how it may be prescribed.

If the drug made subjects euphoric, they are asked whether they would take it again for pleasure, and how much they would pay for it on the street.

The four categories of drugs that cause the most trouble, and are most scrupulously tested, are painkillers, diet and sleeping pills and anti-anxiety drugs, said Dr. John J. Boren of the National Institute of Drug Abuse.

Drug companies have been trying for years to develop painkillers as effective as morphine but not as addictive, Boren said. But pharmacologists have been able to invent new sleeping pills that are less habit-forming than the traditional sort, barbituates, he said.

Abuse-proof drugs are medically desirable. But they can also be immensely profitable, since doctors are likely to prescribe them with a freer hand.

Depending on the type of drug being tested, scientists may recruit college students, prisoners, recovered junkies or people in methadone programs, said FrankVocci, chief of the drug abuse staff of the FDA. Some are regulars.