There were 40 bank robberies in the District in 1985 -- fewer than in any year since 1970, FBI and D.C. police announced yesterday.

The total was 10 fewer than in 1984 and 31 less than the number of bank robberies in 1983, FBI and D.C police officials said at a joint news conference.

Both police and FBI officials attributed the decline and the fact that 36 of the 40 bank robberies were solved to the cooperation between their agencies, along with tips stemming from the Crime Solvers program, and the use of such bank security devices as exploding packets of red dye placed with money handed over to robbers.

The number of robberies in most suburban jurisdictions increased slightly last year, according to the FBI and spokesmen for area police departments.

The only other jurisdiction in the local area that showed a decrease was Fairfax County.

Warren Carmichael, the Fairfax County police spokesman, said the decrease in Fairfax from 23 in 1984 to 17 last year, "was a slight decrease and was not enough to establish a pattern."

In Maryland, Andy Manning, a spokesman for the FBI's Baltimore field office, said the number of bank robberies in Prince George's had remained stable for the last two years at 24, but that Montgomery County had shown an increase from 22 in 1984 to 26 in 1985.

Manning said that, in the past, he has noticed that bank robberies tended to increase in Prince George's and Montgomery counties whenever they decreased in the District.

In Arlington County, police spokesman Tom Bell said that his county had an increase of five bank robberies in 1985 over 1984. He attributed the increase to nonresidents.

"We feel that there has been a general increase in crime in Arlington County since the Metro opened," Bell said. "And with the bank robberies, we find that most of them are from outside the county and often from the city the District ."

Lucy Crockett, public information officer for the city of Alexandria, said there were eight bank robberies there in 1985, as compared with three in 1984.

"We don't know how to explain the increase," she said.

"We are unable to explain increases or decreases in crime in general. There are many factors involved."

The first bank robbery of 1986 in this area took place yesterday in Fairfax County, when the Perpetual American bank at 6324 Richmond Hwy. was robbed by a woman about 9:15 a.m., according to police.

Two suspects subsequently were arrested.

Carmichael said that the police department, assisted by the FBI, was off to "a real fine start" in making the arrests.

Carmicheal said that police officers and FBI agents arrested Dophne M. Eads, 37, of 4143 Savannah Place SE, and Adrian E. Wise, 27, of 1511 Longfellow St., Hyattsville, about 11 a.m. about half a mile from the bank.

Eads and Wise have been charged with armed robbery, he said.

Carmichael said the FBI considers the two to be suspects in a number of other bank robberies in the area.