Washington's transitory population is taking the edge off a growing shortage of mid-priced apartments in Montgomery County, but both luxury and low-income rental units are as scarce as hen's teeth, according to a recent survey.

The survey, released last week by the county's Office of Landlord-Tenant Affairs, found 2.6 percent of the county's 52,259 rental units vacant, compared to a 5.1 percent rate two years ago.

The figures suggest that the rental market is growing tighter, although it has not yet reached the crisis level of the county's last major apartment shortage in 1979, said Nikki McCausland, chief of the landlord-tenant office.

"Clearly, the expensive units are full and the inexpensive units are full. But a lot of turnover in medium-priced units is tending to make them more available," she said.

A vacancy rate of 3 to 5 percent is considered desirable, said McCausland.

Anything below that means "it's going to take people longer to find a unit, and also the rents probably are likely to be higher," said Fran Lunney, an Arlington housing official.

Areawide, the trend appears to be the same.

The vacancy rate in Arlington is 1.4 percent, compared to 2.1 percent in Fairfax County, 2.4 percent in the District and 2.7 percent in Prince George's County, according to Lunney, who recently conducted her own survey.

Since 1980, Washington's supply of apartments has undergone dramatic changes, said Arthur D. Goldstein, an economist in the Department of Housing and Urban Development's regional office.

A decline in new construction and a rash of condominium conversions have cause a "significant inventory loss," said Goldstein.

High interest rates also have limited the number of apartment dwellers who can afford to buy, said Donald Slatton of the Apartment and Office Building Association.

On top of general trends, the arrival of subway sevice has boosted demand for rentals in the suburbs.

"People who traditionally look to the District because of transportation can now consider the suburbs," said Slatton. "I predict the apartment buildings within a short distance of those subway stations are going to see an upsurge in demand."

In Montgomery County, where Metro's Red Line has recently been extended to Shady Grove, apartments already are in scare supply along the line, said McCausland.