Fairfax County, the fastest developing jurisdiction in the metropolitan area, has been forced to reopen its old courthouse jail for its growing population of inmates.

The courthouse jail, which was outmoded and overcrowded, was closed early last year and replaced by a new $5.4 million structure whose barless cells and color-coordinated interiors represented the newest developments in correctional facilities.

But late last week, jail officials having run out of beds at the new facility and unable to borrow more space elsewhere in the state, were forced to unlock the barred cells of the old jail, built in 1953.

After cells were scrubbed and fumigated, they became the new home over the weekend for 27 inmates, including those serving Saturday-Sunday sentences ("weekenders," as they are called).

Opening up the courthouse jail forced authorities to drain manpower from the new jail - six deputies had to be transferred - and, according to Sheriff James A. Swinson, increased the risks of prisoners escaping.

Prisoners have to be transported from jail to jail when they are first incarcerated, and then twice a day for breakfast and dinner. "Every time you move a prisoner," Swinson said, "you increase the chance of an escape."

"I think it's too expensive to administer two operations," said John F. Herrity, chairman of the Fairfax Board of Supervisors. "But i don't know how you can stop it."

When the new jail was proposed, Swinson wanted room for 300 inmates. Herrity was a strong supporter of Swinson, but a majority of the board voted in 1976 for a 198-bed facility.

The board expected the state to finance projects that would divert prisoners into halfway houses and programs that would not require incarceration. But the programs either did not materialize or were not realized in the magnitude expected.

Chief Deputy Sheriff M. Wayne Huggins said that 82 percent of the new jail's prisoners were sentenced for offenses related to drugs or alcohol.

Huggins said the jail has neither the staff nor facilities to treat alcoholics, yet must lock them up - as many as 30 to 40 on a weekend - when they get in trouble.

Swinson said that in one of his first acts as sheriff, in 1966, he advocated creation of a detoxification unit, but no such facility was ever built. Alcoholics can be treated at Fairfax Hospital, but it has room for only 10 patients, and according to Huggins, will not accept inmates if they exhibit violent behavior.

During November and December 1978 and January 1979 the jail held 148 people suffering from drug - or alcohol-induced delirium tremens, Huggins said.

Last December, in a case that drew wide attention, 28-year-old Donald L. Ferguson died of kidney failure after a four-day confinement in the jail. He was the third black inmate to die in the jail in a six-month period. Swinson, who said Ferguson was admitted suffering from delirium tremens, acknowledged that the inmate did not get proper care at the jail.

For more than a year, Fairfax has been sending surplus prisoners - as many as 75 - to jails in other jurisdictions. But Huggins said, those jails are now filled.

In March, he said, the jail logged 17,223 miles on its vans while transporting prisoners to places as distant as Roanoke. Under a directive by the county supervisors, a consultant is studying whether the new jail should be enlarged or the old one renovated as a permanent annex.

Swinson, impatient with waiting, said, "I don't need a consultant to tell me I need a bigger jail. Common sense tells you it's needed."

Fairfax is not the only area jurisdiction running out of room for its prisoners, Overcrowding has been reported almost everywhere except in Arlington.

"We're full," said an Alexandria jail official. "We're turning away people, if that's possible."

He explained that lately authorities have been asking police to retain some prisoners at the headquarters lockup, rather than bring them to the jail, which has room for 115 inmates.