The Drug Enforcement Administration, escalating its war on illegal drugs, announced yesterday that its agents have resumed using herbicides to kill marijuana plants on federal land.

DEA Administrator John C. Lawn said agents began to spray 10,000 mature marijuana plants on five plots of land in a federal forest in the Midwest yesterday. He would not reveal the exact location of the spraying because DEA agents were still in the area.

Lawn said the marijuana is being sprayed with glyphosate, a much less lethal substance than paraquat, the controversial herbicide used by the DEA in marijuana eradication until a court order halted its use nearly two years ago.

Studies on mice have indicated that glyphosate is 29 times less lethal than paraquat.

Lawn described the chemical as a weed killer readily available at stores and said a person would have to smoke more than 139 contaminated marijuana cigarettes a day to endanger one's health.

"The risk factor for citizens is about one in 250 million," he said.

However, Kevin Zeese, national director of the National Organization for the Reform of Marijuana Laws (NORML), said yesterday that paraquat and glyphosate cause cancer and that he plans to file a lawsuit to stop the spraying.

Lawn said that yesterday's spraying was being done manually by agents on the ground and that future use could involve airborne spraying similar to two controversial DEA efforts in 1983.

In those cases, paraquat spraying in national forests in Georgia and Kentucky led to a federal court challenge by environmental groups and the NORML.

Under a consent decree, the DEA agreed not to use paraquat until it completed an environmental-impact study. The finished study concluded that "there is a slight risk that heavy smokers of marijuana could be affected by paraquat-sprayed marijuana."

Lawn did not rule out future use of paraquat, but DEA sources said the agency is waiting for the Environmental Protection Agency to ask manufacturers to change the label on paraquat, which warns against use in wildlife habitats. Because of the timing, the DEA is not expected to use paraquat during the current growing season.

Lawn said DEA would use aerial spraying only on federal land in remote areas where it would not affect humans or endangered species.

"The marijuana cultivators now know they cannot look for a site in the remote parts of the United States," he said.

DEA estimates that marijuana is grown in all 50 states and that about 12 percent of marijuana available in the United States is domestically grown.

Growers have made use of federal lands because they are often in remote, unpopulated areas and law-enforcement officials have difficulty identifying who is cultivating the land.