More than 30 Northern Virginians have been arrested on charges involving the illegal sale of the hallucinogen PCP following a year-long investigation described yesterday by law enforcement authorities as the largest drug sweep in Loudoun County history.

The arrests, carried out Tuesday night, were based on findings of a special grand jury that resulted in 26 felony indictments in connection with conspiring to sell the illicit drug.

Nearly a dozen other persons, several of them juveniles, were indicted on felony and misdemeanor drug charges as a result of a separate investigation by the Loudoun sheriff's department into drug sales on the street and in area high schools.

Most of those arrested were from the Sterling Park, Leesburg and Ashburn areas in the more developed eastern police also arrested several persons living in Vienna and Herndon.

Seven persons were still at large yesterday, authorities said, five of them known to be out of state.

The names of those arrested were not released because local prosecutors requested the indictments be sealed until all warrants could be served.

Loudoun prosecutor Thomas D. Horne yesterday called the arrests significant, but said he doubted they would stop either the manufacture or sale in the county of PCP, described by authorities as "the most dangerous drug on the streets right now."

"It'll just change hands," said Lacy of the county's drug network, "from one group of people to another."

Though the investigation cast a wide net, and kept several investigators undercover for months at a time, Horne said it failed to reveal anyone actively producing the drug, which normally sells wholesale for more than $1,500 a pound.

A contingent of nearly 30 state and county police officers met in the county board room Tuesday evening, shortly after the indictments were delivered. From there they fanned out over the area to make the arrests simultaneously.

No seizures of the drug were made, authorities said, and future prosecutions were expected to rely on evidence other than samples of confiscated PCP. Most of the indictments were obtained, Horne said, as a result of information contained in account books seized in an Ashburn home more than a year ago, following the arrest of two persons on drug charges in Tysons Corner in 1979.

Horne described the PCP network as a "spider web" of individuals acting independently, but also linked in various ways. Unlike more expensive and imported drugs, such as cocaine, PCP typically is made in makeshift home

Horne said one of those who had been under investigation was murdered "execution style" along with his girlfriend while driving his pickup truck on a deserted road near Dulles Airport in November. Those deaths still are under investigation.