Labor Secretary William E. Brock, rejecting strong appeals from organized labor, churches and health groups, has decided not to impose a federal standard to require employers to provide work-site toilets and drinking water for farm workers, according to department sources.

In his first major decision on a controversial regulation, Brock apparently brushed aside pleas from the labor groups he has courted during his five months in office. But his long-awaited announcement, expected later this week, is certain to please the farm lobby, which had argued that a federal standard would be costly, unnecessary, and would interfere with states' rights to regulate agriculture and health.

Although Brock accepts medical evidence that a lack of sanitary facilities contributes to a variety of health problems among farm workers, the secretary decided that state governments, rather than the Occupational Safety and Health Administration, should have primary responsibility for field sanitation, according to a ranking department official who asked not to be identified.

The regulation would have required farmers who employ 11 or more workers to provide sanitation facilities within a quarter-mile of every 20 employes.

The official said Brock will announce that in 18 months he will reconsider his decision. Brock is expected to say that unless more states have enacted -- and enforced -- field sanitation standards by then, the federal government will step in. Currently, 13 states have their own field sanitation standards. "Unless there is more state action, there will be federal action," the official said.

But a move is already afoot in Congress to bypass OSHA and enact a federal law requiring sanitation facilities for farm workers. Rep. William D. Ford (D-Mich.) has already lined up more than 20 cosponsors for such a bill, which he plans to introduce soon.

Brock's decision is the latest chapter in a 13-year saga at OSHA, which has studied but rejected a standard sought by the Migrant Legal Action Program and other groups on behalf of 500,000 farm workers. The AFL-CIO, the American Public Health Association, the U.S. Catholic Conference, and more than 50 other groups and 75 members of Congress had supported a standard. The primary opponent was the American Farm Bureau Federation, which claims a membership of 3 million.

Former OSHA administrator Robert A. Rowland rejected the standard last April, but Brock pledged at his confirmation hearings that same month to review the decision. The Office of Management and Budget opposed a standard, according to Labor Department sources, who said Brock's decision was influenced, but not dictated, by pressure from administration officials.

According to testimony at OSHA hearings, the lack of toilets and clean water for farm workers contributed to unusually high levels of parasitic disease, urinary infection, diarrhea, skin disease and other maladies. Brock was impressed by the evidence, the official said, but believes that some states have made serious efforts to correct the problems and that others should be given the chance to do so.

"This decision is in tune with the president's attempt to get back to a more federalist approach to problem solving . . . with emphasis at the state level," the official said.

Within OSHA, however, "the senior staff is horrified" at Brock's decision, a longtime agency official said. "This decision was seen as a real 'gut-check' for Brock. This problem dates back to the 'Grapes of Wrath', and if we can't do something on this issue, what are we going to do on other issues? There is a good deal of discouragement that he took a political rather than moral route."

AFL-CIO health and safety specialist James Ellenberger called the decision "a surrender" of OSHA's legal responsibilities. He said labor officials would be "mightily distressed because we expected more from Brock than a delaying action in extending to the citizens of this country the rights that civilized people should have."

Chuck Fields, a Farm Bureau spokesman, said he was surprised by reports of Brock's decision, and that farmers had expected Brock to enact a standard because of the clamor from labor and advocacy groups. He said farmers believed that health problems were exaggerated, that workers often don't use field toilets even when offered, and that OSHA's estimate that the regulation would cost less than $1 a day per worker was too low.