High-rise developers hoping to tear down apartment complexes along Arlington's Metrorail corridors must replace any moderately priced units that are destroyed, under a law passed yesterday by the County Board.

The board's action was another step in Arlington's efforts to slow the loss of low-cost housing. In the last decade, about 10,000 units with below-average rents have been demolished or renovated and placed out of the financial reach of tenants with moderate incomes.

The measure provided immediate protection to residents of the Pollard Gardens apartment complex, whose 124 moderately priced units are to be razed to make way for high-rise residences.

The land use change adopted by the board created a "special affordable housing protection planning area" that applies to land in the Metrorail corridor zoned for low-rise residences. Low- and moderate-income apartment units in the district must remain or be replaced on the original property or elsewhere. In return, developers could be eligible for higher densities.

Tenants at Pollard Gardens, a World War II-era complex near the Virginia Square Metrorail stop, reacted happily.

David K. Rathbun, president of the Pollard Gardens tenants association, said the board's action "shows what tenants can do when they're organized." Specifics on tenant relocation are still being negotiated between the residents and the developer, he said.

The plan was greeted warily by a representative of the developer, who praised the idea but said there currently is no provision for tax-exempt financing, tax credits or other government financial aid.

"We're afraid we're being set up to fail," said Martin D. Walsh, who represents Premier Realty Co., the owner of Pollard Gardens, and Tishman Speyer Properties, a development company.

Some residents in the nearby area were unhappy with the plan, preferring a proposal to change the county's land use plan to prevent any high-rise development on several sites in the Virginia Square area.

Rohan Samaraweera, president of the Ballston-Virginia Square Civic Association, said he believed board members "had made up their minds" beforehand and were not open to alternatives.

But county officials said changing the land use plan would reverse the county's longstanding policy of concentrating high-rise development along its Metro corridors. "The community has bought into this for decades," said County Board Chairman Albert C. Eisenberg.