NORTH POTOMAC, Md. -- After seeing his neighbors sell their high-priced homes in just a few days two months ago, Brian Rabin expected a similar experience when he put his family's three-bedroom townhouse in Gaithersburg, Md., on the market in mid-July.

But after almost a month -- and after lowering the asking price by $30,000, to $649,000 -- he had not a single offer. He decided Monday to take it off the market temporarily. "We are finding it frustrating," he said. "We were hoping to sell quickly, but so far that hasn't happened."

From Boston to San Diego to Washington, D.C., there are signs that the red-hot housing market may be cooling. The number of listings is up, sales are down and more sellers are being forced to trim prices or offer incentives to entice buyers.

Real-estate agents say it appears that prices have become so high that more potential buyers are being squeezed out of the market. Buy-and-flip investors, who often purchase homes before they are built and resell them for a tidy profit after construction is completed, are cashing out early -- in some cases before the house is finished. Families who can still afford the high prices are postponing their purchase plans until they can determine whether the current lull merely reflects summer doldrums -- a routine occurrence in the housing market -- or the beginning of the bursting of the housing bubble, which would be to their benefit.

"I am very wary of buying," said John Hiltz, a high-school assistant principal, as he walked through a $665,000 townhouse in Arlington, Va., last weekend. Mr. Hiltz said he fears that if he buys now, he could be buying right before prices fall. "For a first-time buyer, it is very risky."

No one is suggesting that home prices will crash or that sales will dry up. And there are no signs of cooling in a number of major markets, including Los Angeles and New York. Indeed, most housing-industry economists are still forecasting a healthy market nationwide through 2006.

But real-estate agents say they do see evidence that markets in some metropolitan areas are in the process of readjusting from a purely sellers' market to a market that might be more balanced between buyers and sellers. That, they say, could mean prices will level off or rise more modestly in the months ahead -- especially at the high end of the market, where home builders have added substantial supply.

That readjustment is most evident here in metropolitan Washington, which has had one of the strongest housing markets in the country. During the past five years, the region has created 287,000 jobs, one of the fastest-growing employment markets in the nation. Job growth, in turn, has triggered strong demand for housing. Average home prices in the Washington area were $376,000 last year, up 72 percent from 2000. The number of homes selling above $500,000 has risen especially fast, but some are wondering whether there are now too many high-priced properties.

"There is obviously a change in the market," says Paulette Ladas, a real-estate agent in Silver Spring, Md. "Things are taking a bit longer to sell, and we cannot be as aggressive with pricing as we have been in the past."

Ms. Ladas says condos and townhouses are still getting multiple offers, but she has noticed a change in the single-family market. "That tells me that this is a slowdown because it is starting in the upper brackets," she says.

A Wachovia Securities LLC analyst cited slowing sales in the Washington area as one reason to downgrade the stock of luxury-home builder Toll Brothers Inc., Horsham, Pa., which has a number of developments under construction in the region. "Rising prices have made it more difficult to find quality buyers," wrote Carl Reichardt, the analyst.

Ron Cathell, an agent with Weichert Realtors, says his open houses, even in the dead of a hot, humid Virginia summer, can draw as many as 60 people. Business was slower, however, last Sunday, when he says only around 20 people showed up to view a $665,000 townhouse in Arlington. "I haven't seen an open house this slow since 1998," he says.

Among Washington's suburbs, inventories of homes available for sale are growing fastest in northern Virginia, which had about 4,843 homes listed in July, up 26 percent from a year earlier, according to the Northern Virginia Association of Realtors.

This past Sunday in Sully Manor, a Centreville, Va., development filled with new homes and townhouses priced as high as $825,000, "For Sale" signs sprouted from at least six yards on one street alone. Prospective buyers driving Lexuses and Mercedes-Benzes drove up and down the street with puzzled expressions on their faces. "The ("For Sale") signs make people nervous," says Chan Lee, a Vienna, Va., real-estate agent who is selling one of homes on that block. "You want to buy what everyone else is buying because you think it's good -- but if no one is buying anything, that's bad."

Most of the Sully Manor sellers are investors who purchased their units early on and have never lived in them, a local sales agent says. Their properties have appreciated, so they should profit from the sale even though they are asking less than the builders' list price, which has gone up.

Investors are putting their properties on the market mostly out of fear, says Sue Ko, an Arlington real-estate agent. "People are hearing about interest rates and a lot of other issues they don't always fully understand, so they're considering their exit strategies," Ms. Ko says. "They're getting out of it before the bubble bursts."

One of the builders at Sully Manor, Atlanta-based Beazer Homes USA Inc., is trying to attract buyers by offering as much as $30,000 in options, upgrades and incentives. Beazer is also reaching out to northern Virginia real-estate agents by inviting them to a "Summer Realtor Appreciation Luncheon" later this month so they can check out its properties in the area.

Individual investors in the area also are offering incentives. Manish Paliwal, a 35-year-old Internet analyst from Ashburn, Va., thought the $676,000 home he bought in Lansdowne, Va., in February was a good investment property. But since the market seems to have slowed down, he's not so sure. Not only has he listed the home for $699,000 -- less than the $750,000 at which it was recently appraised -- but he also is offering the agent who successfully sells his home an additional $2,000. "There's a lot of competition out there," he says.