For 285 Fairfax County residents, the best of times has meant the worst of times.

The Washington area's thriving economy, with its low unemployment and high salaries, has brought trouble to the Northern Virginia Training Center, a state facility that is home to 285 mentally retarded children and adults.

Their caretakers are quitting, lured elsewhere by better pay and easier work. The average length of stay for direct-care aides at the Braddock Road facility is nine months, six of which are spent in on-the-job training, according to David Lawson, a psychologist and director of the center. Seventy percent of the center's 160 aides do not last a year, well above the current national job turnover rate of about 13 percent annually, as measured by the Bureau of National Affairs.

High turnover among center aides means that the residents' surrogate family is constantly changing, further disrupting their already confused world. The residents, some of whom also have mental and physical illnesses and most of whom function on the mental level of 1- or 2-year-olds, are fed, dressed, supervised and taught skills by the aides.

The high turnover also means that the state and federal governments are spending increasing sums on hiring and recruiting aides -- it takes $5,000 and six months to train each worker -- and it means that abuse of residents is more likely to occur because new aides are more likely to become frustrated with patients and lash back at them, and less likely to detect abuse by coworkers, according to mental health professionals.

The Justice Department is investigating allegations that two aides at the center physically abused more than 20 residents last year. Those aides have been fired.

"We feel the abuse problem we had there a year ago is directly related to the ability to keep people," said state mental health commissioner Howard M. Cullum.

Cullum has asked the Virginia General Assembly for an immediate infusion of $1.9 million to boost the aides' salaries, which start at $13,400, or $6.40 an hour. The General Assembly is considering Cullum's request.

"Sometimes around here, we hope for a recession," said Lawson. With hotels, restaurants and offices often offering around $7 an hour for less demanding work, Lawson said he understands why aides leave. "They could go to McDonald's and not have the hassle."

Ellen Kotellos started working at the center six months ago and already has seen many on her shift go. "The number one thing I'm concerned about is that the residents get used to one person," she said. "They depend on you for tender loving care." Aides "become like their parents and then they leave."

The exodus of mental health workers is not confined to Northern Virginia.

Forest Haven, the District's residential facility for the mentally retarded, has had major problems keeping staff members. A judge, citing understaffing among other problems, ordered the facility shut down, and residents are being transferred to group homes in the community.

In Maryland, Great Oaks Center, a Silver Spring facility for the mentally retarded, has been threatened with loss of federal funding three times since 1985 because, among other reasons, it did not have enough staff.

Since then, Great Oaks has added 50 direct-care workers, dropped its population by 100 and started a three-year program to increase wages from $4.50 to $6 an hour, according to Maryland mental health officials.

Marvin Malcotti, director of Great Oaks, said that since the staff has been increased, the number of employee suspensions has been drastically reduced. In 1986, he said, there were 274 suspensions. In May and June 1987, the last time suspensions were counted, there were four.

Malcotti said he could not detail each suspension, but that some were for abuse of residents.

Mental health professionals say that the fewer the workers, the higher the workload and stress level, and therefore the likelihood of quitting. This cycle is not new, they say, and the problem has long been ignored.

Historically, they say, mental retardation problems have not received as much public attention as other social service needs; of late, attention has focused on deinstitutionalization.

Recent threats by the federal government to withdraw Medicaid funds from understaffed centers has drummed up more state and local interest, mental health advocates say, but the problems persist, in part because the residents are not a vocal constituency for change and because the government's deep pockets can afford the costs associated with high employee turnover.

Sixty-three of the 450 mental health facilities inspected by the federal Health Care Financing Administration in fiscal 1987 did not meet staffing or other federal regulations and were threatened with loss of federal funding, which often amounts to half their budgets.

"If there are not enough people and they don't know what they are doing, accidents and injuries can happen," said R. Wayne Smith, director of the agency's Office of Survey and Certification.

The worst recent case of understaffing occurred at Fairview Training Center in Salem, Ore., according to agency officials.

"There were a significant number of accidents and injuries" there, Smith said. "People were heavily medicated, suicide attempts were made, there was a sexual incident . . . . It was a very dramatic example of what the national shortage can lead to."

The agency stopped Medicaid payments to Fairview last April and ordered the center to increase its staff and reduce its population. The facility, which lost $8.1 million in federal funding, has since hired 700 employees for a total staff of 2,100, according to Peggy Sand, spokeswoman for the Oregon mental health department.

She said Fairview also plans to reduce its population from 1,080 to 780 during the next two years.

Joseph Gaffney, a developmental disability specialist with the agency, said that few people realize the importance of having a reliable, steady nucleus of aides at mental health facilities.

They understand residents' behavior, react better to difficult situations, and are not as easily provoked by angry residents, he said.

"All across the country, because of insufficient staff, residents have injured other residents, mutilated themselves -- residents have even disappeared," he said.

Nationally, the federal and state governments spent more than $1.7 billion on salaries for direct-care employees at public facilities for the mentally retarded in 1985, according to Alfred A. Baumeister, director of Vanderbilt University's John F. Kennedy Center for Research on Mental Retardation and Human Development.

Increasing turnover means that increasing amounts will be spent on recruiting, hiring and replacing staff instead of improving it, he said.

"It's costing taxpayers millions of dollars, and it's solvable," Baumeister said, by providing day care for workers' children, more flexible work hours and merit pay raises.

While Cullum, Virginia's mental health commissioner, said a higher wage scale is "no easy panacea, it is the most visible way to reduce turnover."

Others said there should be heightened efforts to interest those studying psychology, social work or medicine in working with the mentally retarded. Since mental health workers are needed around the clock, efforts are being made to recruit those willing or forced to work odd hours.

Perhaps most important, Gaffney said, those who care for the mentally retarded must be valued more in society.

"It is a matter of perceptions and national priorities," he said. "I know saints who would get bounced out of heaven . . . by some of these direct-care aides."

"What makes me stick with it," said Larry Compton, a 23-year-old aide at the center, "is the residents.

"They like to sit in the day room and talk. They can say off-the-wall things and the most incredibly sensible things . . . . I consider a lot of them my friends."