The number of crimes reported in Montgomery and Prince George's counties increased or remained stable in every major category except rape during the first nine months of 1985 compared with the same period in 1984, Maryland State Police reported.

The greatest increase was in the number of car thefts, which rose by 27 percent in Prince George's County and 21 percent in Montgomery County from January through September, compared with the same period in 1984, according to the state police statistics that were issued this week.

The only decrease was in the number of reported forcible rapes in the two counties, which were down 12 percent in Prince George's and4 percent in Montgomery.

Overall, however, violent crime, including homicide, robbery and aggravated assault, increased 11 percent in Prince George's and9 percent in Montgomery.

There were 32 murders in Prince George's County in the first nine months of 1985, compared with 28 last year, while in Montgomery County there were nine murders during that period in both 1984 and 1985.

"Rising crime statistics are generally reflective of suburban counties throughout the Washington area," said Prince George's County Police Chief Michael Flaherty.

Much of the change, he said, is because the number of persons in the "crime-prone years of 14-24" is increasing.

While crime in Prince George's County is up since last year, Flaherty said, it still is lower than five years ago.

In 1984, the crime rate in the county was the lowest since 1975, Flaherty said.

State police spokesman Chuck Jackson said that car theft rose substantially in all areas of the state except western Maryland.

"It's something we're definitely concerned about," Jackson said. "We compile these statistics to give direction to our field operations, and we may redirect our resources toward combating theft of motor vehicles."

Capt. Dave Mitchell, a spokesman for the Prince George's County police, credited the drop in the number of rapes to increased police surveillance after a series of rapes last spring in the Riverdale area and near the New Carrollton Metro station.

County police "targeted that crime and made a number of apprehensions," he said, adding: "That has certainly helped."

While the number of rapes is down in Prince George's and Montgomery counties, rape was up 6.8 percent statewide.

Jackson and other police officials cautioned, however, that reading too much into crime statistics can be risky.

"The only thing we can do is read trends, what's going up and where, and concentrate our efforts on those areas," said Phillip Caswell, spokesman for the Montgomery County police.