A truckload of ether was the first marker on the trail that led federal investigators to a remote cocaine processing laboratory here in rural Orange County, Va., last week, where they arrested six persons and seized finished cocaine worth as much as $20 million.

Drug Enforcement Administration agents said they had been probing a suspected cocaine processing cooperative in New York for about two years and were following the trail of chemicals used to make finished cocaine. On June 5, a Ryder rental truck, containing 30 drums of ether, headed south from Long Island and was later stopped by an unsuspecting Maryland state trooper as part of a routine check.

When their investigation ended last Wednesday in Orange County, federal, state and local officials seized 70 unopened 55-gallon drums of ether, acetone, acids and other chemicals used in the manufacture of cocaine on the 250-acre farm, as well as more than 40 large trash cans containing chemicals and cocaine in various stages of processing plus 70 pounds of the finished drug.

"To our knowledge, there has never been this quantity of cocaine-producing materials seized in Virginia," Orange County Assistant Commonwealth's Attorney Charles W. Bowman said last week.

"We certainly weren't anticipating anything of that size," said Commonwealth's Attorney Christopher J. Honenberger.

Last Thursday, authorities in New York raided two similar laboratories there; the three seizures, they said, broke up a major East Coast cocaine processing operation allegedly run by three Colombian families.

It is the kind of investigation that has occupied drug enforcement agents increasingly in recent years, as soaring prices for ether and acetone in South America have driven cocaine processing operations to remote locations in this country.

And it would be difficult to find a location much more remote than the small town of Gordonsville, population 1,500, in Orange County.

The 355 square miles of Orange County are still mostly crop land, pasture and forest, with only 4 percent of the area commercially developed. Fourteen times a day, the cars on Main Street idle their engines while a train clatters through town.

"It's a fairly new phenomenon," said William Schnepper, a DEA spokesman who described the Gordonsville raid as the "first, only and biggest" such seizure in Virginia.

In the last several years, the price of a 55-gallon drum of ether in South America has rocketed from $800 to $7,000, while the same amount costs $500 to $600 in the United States, according to Andrew Fenrich, another DEA spokesman.

At the same time, the number of cocaine laboratories seized by DEA agents has risen each year, from three in 1980 to 11 in 1983.

Because the chemical reactions involved in cocaine processing create a sharp, unpleasant odor, the finishing laboratories tend to be located mostly in remote rural locations to prevent authorities from literally sniffing them out, a DEA spokesman said.

According to interviews with DEA agents and an affidavit for a search warrant filed in Orange County Circuit Court, the rental truck was stopped by a Maryland state trooper for a routine equipment violation; during a casual conversation, the driver told the officer he was headed for Orange County.

Orange County officials, alerted by Washington DEA agents, searched in vain for the truck. The next day, according to the affidavit, another call came from federal authorities: They told Orange County investigator William Spence to look out for a farm that had recently changed hands.

"At this point, it was emphasized [to Spence] that these participants were a very dangerous group of individuals capable of murder and would be heavily armed," the affidavit says.

Tax statements showed that Robert M. Cadiz, formerly of Falls Church, had bought the Gordonsville farm for $160,000 and had moved in May 4, Spence said in the affidavit. Cadiz was one of the six arrested last week and charged with possession of and/or manufacture of cocaine.

On July 5, Orange County sheriff's deputies had begun to watch the farm, meticulously recording the movement of trucks in and out, the installation of exhaust vents on top of the metal shed and the unloading of 55-gallon barrels, according to the affidavit.

The surveillance continued, in 12-hour shifts, for four days. At 5:30 a.m. on July 10, authorities raided the farm, seizing the processed cocaine, the chemicals and several weapons, including an automatic pistol and a military-style carbine rifle.

The raid was big news to federal drug enforcement authorities, who said the three raids broke up the first cooperative drug processing arrangement known in the Northeast.

But it was bigger news 82 miles southwest of Washington, where word of the giant cocaine raid jarred the slow rhythms of rural life in this central Piedmont county of 18,000.

For a week before the raid, the quiet routine of law enforcement in Orange County -- where General District Court meets only once a week -- was replaced by the feverish pace of a major drug investigation.

Virtually the entire investigative unit of the sheriff's department -- nine deputies, the sheriff and the captain -- were involved in the surveillance.

After the raid, Main Street in Orange buzzed for days with gossip about the cocaine lab; phones in the commonwealth's attorney's office rang constantly, and visitors who had never heard of Orange County wandered through to gawk at the farmhouse on the hill.

"Yeah, we've kind of been talking about it right much. Sounds like it was a right big event," said Jack Samuels, owner of the Cinema Plus video rental store on Main Street.

"In Orange County, we don't lock our cars, we don't lock our houses, and we don't expect a lot of heavy drug-smuggling going on," Samuels said.

Confronted with sudden notoriety, citizens of Orange County greeted the news with a mixture of surprise, curiosity and relief.

"There have been rumors around here for a couple of years now about people selling and dealing cocaine, but it was just rumors -- nothing was proved," said Sally McClain as she nursed a beer in DeVivi's restaurant near Gordonsville. "And then . . . there it is. I'm sure glad they found it." CAPTION: Picture, Virginia state police inventoried findings last week at a farmhouse at Gordonsville, four miles from Orange, that they said was used to process cocaine. An estimated $20 million of cocaine was found and six people were arrested in the raid. AP