Fairfax County's Board of Supervisors, fearful of being inundated by drug paraphernalia merchants from Maryland, yesterday enacted their first measure aimed at limiting the sale of such devices.

But what they didn't know was that only hours earlier a federal appeals court in Cincinnati had dumped cold water on key provisions of their new legislation.

In a unanimous decision, a three-judge panel of the 6th Circuit Court of Appeals in Cincinnati struck down the Justice Department's model paraphernalia law that Fairfax had used as the basis of its attack on the sale of accessories to illegal drug use such as pipes, scales, needles and blenders.

While the Ohio ruling does not apply to jurisdictions outside the Midwest, it virtually assures that the model statute will be appealed to the Supreme Court, and could jeopardize the fate of similar politically popular laws in 11 other states including Maryland.

It was only after a federal district court judge recently approved of Maryland's drug paraphernalia law that pressure began to mount on Virginia officials for similar legislation. The Fairfax board acted yesterday after some residents voiced fears that the sellers of drug devices in the Maryland suburbs would simply move across the Potomac and into Virginia.

Under the Fairfax measure, passed unanimously on an emergency basis, paraphernalia merchants wishing to do business in the county will be limited in the future to major shopping centers like Tysons Corners and Fair Oaks Mall. The county's existing paraphernalia stores, believed by police to number less than 10, would not be affected by the measure, an amendment to the county's zoning ordinance.

The county's new measure depends in large part on definitions of drug paraphernalia that were taken from the federal model law that was struck down by the court in Cincinnati. In examining three cleveland-area ordinances, the Ohio panel was particularly critical of the law's attempt to define exactly what constitutes drug paraphernalia.

"Our opinion questions not the laudable goals of the city's elected officials, but the means chosen to implement their goals," the court wrote. "We are most cognizant of the serious problem of drug abuse long existing in our nation. We hold only that the drug paraphernalia ordinances fail the test of precision demanded" by the Constitution in the areas of free speech and due process.

The Fairfax zoning measure was aimed at keeping such stores away from local neighborhoods where they might influence young children, said Supervisor Audrey Moore (D-Annandale), who sponsored the measure. Under current Virginia law, drug paraphernalia shops are prohibited from selling their wares to customers younger than 18.

Peter H. Meyers, chief counsel of the National Organization for the Reform of Marijuana Laws, yesterday called the Cincinnati decision a "landmark case" because it represents the highest court that has yet considered the constitutionality of such laws. "This is a persuasive authority, and we would expect many other courts to follow this decision," he said.

Peter Bensinger, director of the Justice Department's Drug Enforcement Administration, said he was "not unnecessarily gloomy" about the fate of the model, which his agency authored. He said he was hopeful that appeals court judges in Richmond, St. Louis and New Orleans would side with the 15 lower court judges around the country who have previously upheld the statute.

Virginia delegate Larry Pratt (R-Fairfax), who unsuccessfully sponsored a drug paraphernalia bill before the General Assembly in Richmond earlier this year, said yesterday the Cincinnati decision does not dampen his hope for victory at next year's session. "I don't think this one decision is a setback," he said. "I think we're still ahead of the game."