The Montgomery County Council, responding to increasing concern over the effects of pesticides, enacted emergency legislation yesterday requiring lawn care companies to post warning signs when they battle weeds and outdoor pests with toxic chemicals.

The unanimous vote by the seven-member council capped more than two years of debate and unsuccessful negotiations with the lawn care industry to establish a voluntary pesticide warning program.

Stewart McKenzie, an aide to the council on environmental affairs, said, "You're dealing with some potent chemicals here and some unknowns about their health hazards. It's just prudent to do."

McKenzie said the program would help protect people who are hypersensitive to pesticides as well as pregnant women and infants, who run a substantially higher risk of ill health from chemical exposure.

By passing the bill, however, the council may have set itself up for a lawsuit. In an opinion last October, Maryland Attorney General Stephen H. Sachs warned that the council was acting in an area of law preempted by state and federal government.

Yesterday council members brushed those concerns aside. "This is not a pesticide bill, this is a right-to-know, consumer safety bill," said council member Esther P. Gelman, a longtime supporter of the idea.

Lawn companies that oppose the measure have argued that when the chemicals are professionally applied they present no danger. They said warning requirements are burdensome.

The bill would take effect in April to coincide with this year's growing season. It will require anyone who applies pesticides and herbicides to lawns commercially to inform customers of the types of chemicals used and to post signs identifying where they have been applied. The bill does not set a time limit for removal of the signs.

More than 140 local stores that sell the chemicals also would be required to provide customers with information and signs, but homeowners applying the chemicals themselves are not required to post the warnings.

Violators would be subject to a maximum $50 fine and 10 days in jail, according to the bill.

The council members in their discussions pointed to a study by the National Institutes of Health that found higher than normal rates of lung cancer, leukemia and brain cancer among 3,800 commercial pesticide applicators in Florida.

County Executive Charles W. Gilchrist agreed in 1983 to sponsor a measure on behalf of county garden clubs and the Audubon Naturalist Society, but first urged the county's Department of Environmental Protection and Office of Consumer Affairs to try negotiating a voluntary agreement.

Those negotiations collapsed last year after lawn care companies refused to post signs.

Gilchrist put the bill in for council consideration last year.

The county has been posting public property treated with chemicals since last year, and the Prince George's County Council enacted a strict pesticide notice law last October, according to a council memorandum. The Prince George's law requires anyone using pesticides on lawns, golf courses and rights of way to post warning notices when chemicals are sprayed.