Alexandria and Arlington police yesterday began arresting 42 persons on drug charges after a six-month undercover investigation of cocaine and marijuana sales at several Northern Virginia nightclubs and restaurants.

Police said they confiscated about $68,000 worth of cocaine and marijuana during the investigation, which was prompted by complaints from patrons of several area restaurants and bars of widespread drug trafficking.

"Cocaine is rampant in Northern Virginia," said Lt. Arthur L. Bratcher of the Alexandria Police Department, which conducted the investigation with the Arlington Police Department.

Police said they issued warrants yesterday for the arrests of 42 persons on charges of selling or attempting to sell cocaine or marijuana to undercover police officers. Police said they expected to have arrested about half of the individuals by last night.

The investigation included bars in Old Town and the West End in Alexandria and establishments in Crystal City, Columbia Pike and Rosslyn in Arlington, police said.

Alexandria spokeswoman Lucy Crockett said she would not disclose the names of the bars and restaurants because "there is no indication from the investigation that management or employes of any of the restaurants were involved in the distribution of drugs."

Cocaine is "currently the drug in vogue," said Lt. John S. Karinshak of the Arlington Police Department. "It's very prevalent . . . . We didn't target any socioeconomic groups. We targeted those in the age group between 20 and 35."

"I'm seeing an increasing number of cocaine cases coming across my desk," said Alexandria Assistant Commonwealth's Attorney Randolph J. Myers, who prosecutes most of the city's adult felony drug cases.

If convicted, those charged with distributing cocaine could be sentenced to a maximum of 40 years in prison and fined $25,000, Myers said.