Business was slow yesterday afternoon in the El Dorado Health Club, a two-story brick building set in a row of boutiques, shops and offices in a trendy commercial block in Alexandria's Old Town.

Only a well-dressed young man sat in the club's waiting room, watching a soap opera on a portable television set until a tall, slender blond woman ushered him upstairs for a "session."

At 6 p.m. today the club, at 624 N. Washington St., is expected to close because, as Alexandria Assistant Commonwealth's Attorney S. Randolph Sengel argued in court documents, it is actually a "blatant house of ill repute . . . assignation and prostitution."

After nearly a year-long police investigation and a five-hour trial Thursday in Alexandria Circuit Court, Judge Wiley R. Wright Jr. issued a permanent injunction that orders the club closed as a public nuisance. Moreover, the injunction orders that the building be closed to any use for a year, unless its owner pledges to the court that a similar business won't operate on his property.

"It's an extraordinary remedy," said Sengel, who prosecuted the city's case. "This type of action recently has been used successfully against several places in Fairfax County."

Alexandria Commonwealth's Attorney John E. Kloch said yesterday that the El Dorado club case represents the first time the city sought to shut down an alleged prostitution operation by using an 83-year-old state law that terms such establishments a "public nuisance."

Kloch said he plans to dust off the old civil law to go after any Alexandria houses of prostitution thinly veiled as massage parlors or health clubs.

Evidence presented by prosecutors at the trial that the El Dorado club offered sex for money included testimony and sworn statements from customers who said they had bought sex from female club employes. Sengel said there are no plans to criminally prosecute Luis G. Calvo, owner of the El Dorado.

Calvo's attorney, Frederick W. Ford, said his client will close the club, though he added that he didn't think the court treated the matter fairly. He said the city never proved that Calvo knew that any of his eight or so female employes were engaging in sex with customers for pay.

"This is a momumental waste of taxpayers' money . . . to find out that on a couple of occasions two consenting adults in private did something with each other that did no harm to anyone in the community," Ford said.

Alexandria has a history of prostitution rings flourishing openly, some behind the more than 20 massage parlors that once dotted the city. Since the early 1970s, the parlors became the targets of federal and local probes, City Council legislation banning massages between the sexes, and complex legal battles.

"It's our intention to close them all down; those that are essentially houses for illicit sex," Kloch said. "It's been a longtime battle to get them out of the city."

Earlier this week, Charmine's Health Club at 3907 Bruce St., which authorities said is one of the few remaining Alexandria massage parlors under police investigation, filed a suit to halt the probe. An Alexandria Circuit Court judge ruled that police could continue their investigation.