High mortgage interest rates and doubts about the economy may be cooling the nation's home-buying fever but they appear to be having the reverse effect on small-scale investors in rental houses and condominiums.

Brokers in major markets around the country report growing demand by two-income families, single professionals and retirees for small residential investment properties that can be rented out -- especially single-family houses and condominium apartments.

The upsurge in demand for those properties is running counter to the slump in the rest of the market, they suggest, because purchasers are learning how to avoid conventional lending sources and find bargains that didn't exist one or two years ago. The current economy is flattening out resale home prices in many areas and pushing up rent levels -- creating an ideal atmosphere for savvy purchasers.

Brokers in areas as diverse as Chicago, Spokane, Phoenix, this Washington area, south and central Florida, San Antonio and Houston report they can sell as many moderate-priced, rentable homes and condos to small investors as they can find for sale.

Buyers in the middle-income brackets and upward are convinced that there are long-term tax shelter and capital gains opportunities in real estate. So they're plunging ahead with transactions, banking on future appreciation and continued shortages of rental apartments.

"Things are as strong in our segment of the market as they were 18 months ago" at the height of the condo conversion craze, according to Michael Kobi, vice president of Century 21-Rohter Inc., a downtown Chicago investor-oriented brokerage firm.

Kobi said sales volume has been running at about a $40 million annual rate for his 12-person firm -- the largest chunk of it on small-scale condominium apartment transactions financed outside conventional lending sources.

Arlington broker A.M. Barr, a longtime specialist in investor rental homes in Northern Virginia and the Washington suburbs, says the present economy "has doubled or tripled our volume rather than turned investors off."

Barr says small detached suburban homes and town houses in the $60,000 to $70,000 range are being purchased as soon as they hit the market by "people who already own a home or two and know the tax advantages."

Most of the units involve assumable FHA or VA mortgages plus secondary financing in some form. The seondary loans -- designed to reduce the down payment requirements for buyers -- come either from the sellers of the property or from entrepreneurs like Barr himself. He sells participations in pools of second-mortgage money to investor clients.

Virtually none of the rental-home sales involves conventionl lenders like S&Ls or banks, whose loans for investors in the current crunch are either too costly (16 percent to 17 percent) or simply non-existent.

The pattern is similar in Houston, Phoenix, Tucson Florida and other areas. James Crowley, president of Houston's Crowley/Sprinkle Inc., is averaging $1 million to $2 million a month in small-scale investor purchases of rental homes in the $60,000 range.

With rental unit vacancy rates exceptionally low in prime sections of the city and suburbs, many prospective home buyers are forced out of the market by the dearth of conventional mortgage money. "The hottest game in town has become single-family rental homes," says Crowley.

His firm searches Houston's sprawling subdivisions for homeowners who bought their units with low-down-payment, assumable VA loans and now need to move. He convinces them to sell to his clientele of small investors and then arranges secondary financing to cut the investors' down payment costs.

Most of his clients go into purchases with less than 10 percent down, and a growing percentage are able to finance units with almost nothing down. "They know that their $60,000 house is going to be worth $70,000 when the money market bounces back," says Crawley, and their $2,000 to $5,000 cash investment can produce rapid 100 to 300 percent dollar-on-dollar returns.

In Florida, where the real estate market is healthier than in most parts of the country, rental investments are brisk. Fort Lauderdale broker John Ring tells of an 89-unit condominium in nearby Deerfield in which many of the sales have been to buyers who never have invested in rental property before. The one- and two-bedroom units sell quickly for $57,00 to $61,000.

Ring's firm rents out and manages the condo apartments for purchasers. He says he can produce close to breakeven monthly cash flow for the owners with current rents.

Central Florida subdivisions near growth cities like Tampa and Clearwater also have become hunting grounds for local and out-of-state rental home investors. Well-located houses with assumable FHA or VA mortgages are being touted in northern markets as recession-proof opportunities to own a tax-sheltered piece of the Sun Belt.