The 27-year-old Washington government worker remembers the incident clearly. She was returning home from a date, and was walking to the side entrance of her apartment building in Fairfax County, when an arm suddenly locked around her neck.

"I thought someone was pulling a joke," she recalls. "I couldn't scream or pull away."

Her assailant forced her to lie down between two parked cars, robbed her of $15 and raped her at knifepoint.

"For the first two months after it I was a complete basket case," she says. "I'm still . . . not recovered from it. I just have those flashbacks."

That incident is just one of the many rapes reported to suburban Virginia police during the first half of this year. While statistics show a drop in the number of inner-city rapes, police in all but two Washington suburbs say that the number of reported sexual assaults is on the increase.

Nowhere has the increase been greater than in Fairfax County, where police report a 65 percent increase in the number of reported rapes and attempted rapes for the first half of 1981.

Fairfax police spokesman Warren Carmichael said 61 cases were reported between January and June, compared with 37 during the same period last year.

Rape tends to be a "pretty cyclical" crime, Carmichael said, noting that the number of rape incidents in the county has fluctuated over the past three years.

"One of the reasons for it being cyclical is that rape is a crime in which an individual who perpetrates it doesn't always perpetrate just one."

Most of the 32 cases that have been cleared by the department this year, he said, involved the arrest of individuals who are suspected of being multiple offenders. Arrests of such persons, Carmichael said, typically reduce the number of assaults the following the year.

In Prince William County, the number of reported rapes -- 12 -- doubled during the first six months of 1981 compared with the same period last year when six rapes were reported, according to police officials.

In Alexandria, 31 rapes or attempted rapes were reported from January to June this year compared with 27 for the same period a year ago, an increase of 14.8 percent. And in Arlington, police spokesmen say, 12 sexual attacks were reported for the first six months of this year compared with 10 a year ago, a 20 percent increase.

In Northern Virginia, only Loudoun County reported a decrease in rape reports, from five in the first six months of 1980 to three this year. sub

While the number of reported rapes has increased in most Northern Virginia areas, the number of cases that have been cleared has decreased in at least two counties. During the first six months of 1981, the Arlington Police Department cleared 13 cases compared to 16 in the same period last year. The Alexandria department resolved 8 cases this year and 14 in the first six months of 1980. In Fairfax County, however, the 32 cases that have been cleared this year represent an increase over last year, when 22 cases were cleared during the same period.

Police departments declare a case cleared when a suspect is arrested or when an alleged assailant is identified but is not prosecuted by choice of the victim or government attorneys.

In suburban Maryland, Montgomery County police have received 76 reports of rape or attempted rape through the first half of 1981, compared with 65 during the same period last year, a police spokesman said.

Prince George's County and the District reported the greatest number of rapes in the metropolitan area for the first half of this year, although officials in both jurisdictions say those figures represent a decrease over the same period last year.

In Prince George's, police say 172 reports of rape were filed from January to June this year, compared with 187 for the same period last year.

In the District, the number of rapes or attempted rapes reported for the first half of 1981 was 190, compared with 204 for the same period a year ago.

Sgt. Joseph Satterfield of the D.C. Police Sex Squad could cite no particular reason for the apparent increase in suburban rapes. In any event, he and other area officials agree that rape remains "a grossly underreported crime."

Dr. Mace Summers, of Montgomery County's Sexual Assault Services, said the increase in reports doesn't necessarily reflect more rapes, but could mean that women in the suburbs are becoming "more comfortable" about reporting them because of the increased availability of counseling services and victim aid programs.