The Labor Department, reversing earlier plans, announced yesterday it will not issue rules requiring farmers to provide portable field toilets and drinking water for farm laborers.

The department justified the decision by saying that had it set sanitation standards, that the rules would have been assigned a low enforcement priority in any case, and that most farm workers now are protected by state standards. Officials indicated last year that a federal standard would be proposed.

The ruling drew an immediate protest from the Migrant Legal Action Program, whose 1972 petition seeking federal standards set off a running legal and regulatory battle leading to yesterday's announcement.

"We will continue to pursue this because we are not aware of any medical or scientific rationale for rejecting standards," said Luis Torres of the farm worker organization. "The evidence is overwhelmingly to the contrary."

Torres said that while 13 states have sanitation standards, evidence collected by Congress and the Occupational Safety and Health Administration indicated that compliance is low and enforcement scant.

The Migrant Legal Action Program, other labor groups and medical investigators have argued for years that the absence of field toilets and water supplies for drinking and hand-washing contributes to high rates of sickness among farm laborers.

OSHA's explanation of its decision yesterday emphasized that the agency has assigned low priority to the enforcement of sanitation standards for industry in general and that field worker sanitation would get the same "deemphasis."

It said that a sanitation standard also would "reduce scarce resources available to enforce higher priority standards."