The Department of Agriculture, siding with a coalition of conservation groups, has ordered the Forest Service to begin revising one of its most controversial practices: money-losing sales of federally owned timber to private lumber companies.

Responding to an appeal filed by the National Resources Defense Council and other environmental lobbying groups, Deputy Assistant Secretary Douglas W. MacCleery has ordered the Forest Service to rewrite its management plans for three big western forests.

MacCleery said the plans provide inadequate economic justification for selling the timber at deficit prices.

The decision -- reversing a ruling by Forest Service Chief Max R. Peterson which upheld the below-cost sales in the three western forests -- represents the first time USDA has sided with critics of the red-ink timber-sale policy.

Each year, the Forest Service sells tens of thousands of acres of trees to private lumber companies large and small from coast to coast. In some areas -- particularly the timber-rich Pacific Northwest -- the government makes large profits on these sales.

But in other regions -- particularly the Rocky Mountain West and Alaska -- trees in the National Forests are sold for prices so low that the government's costs for road-building, paper work, and other expenses are higher than the amount it gets for the trees.

With losses from these sales exceeding $100 million in some years, the deficit timber selling has been criticized by some budget-minded members of Congress as well as the environmentalists.

But other members of Congress -- including many western conservatives harshly critical of the overall federal deficit -- defend the red-ink policy, which is an important source of timber industry jobs in the rural western counties they represent.

The Forest Service and its parent agency, USDA, have also defended the money-losing timber sales. Forest Service officials say the intangible gains of timber-cutting -- improved recreation, wildlife habitat, and diversity of species in the forests -- make up for the lost millions.

Two years ago, when the service filed proposed long-term management plans for the San Juan, Grand Mesa-Uncompahgre and Gunnison National Forests in Western Colorado, the National Resources Defense Council challenged the plans. The organization said the deficit timber sales violate federal forest laws.

Peterson, of the Forest Service, rejected the environmentalists' challenge. But now MacCleery has sided partly with the challengers.