Marijuana smokers in the United States may be smoking in highly toxic herbicide without knowing it, and the National Institute of Drug Abuse is investigating to see if that is the case.

The herbicide purcuat is a highly poisonous chemical tht Mexico is spraying on its marijuana fields as part of a program helped by American technical assistance and equipment to depose those fields and reduce the flow of allegas germs into this country.

Sen. Charles H. Percy (R-Ill.) first voiced concern over use of the chemical in letters to the State Department and Dr. Peter G. Bourne, director of the White House office of drug abuse policy.

"As you know, Percy told Bourne in a July 18 letter thousands of pound of marijuana come across the Mexican horder every month and there is good reason to believe that a large portion of this marijuana, destined for consumption by U.S. citizens may be tained with the highly poisonous chemical."

As a result of Percy's interest the National Institute of Drug Abuse began a $25,000 research programe to see whether marijuana coming into the country contains paraquat and whether it could be injurious to the health of drug users.

The institute has not concluded its research so it is too soon to make generallgations about the presence of paraquat in this country of its effect.

"Obviously, it's enough of a problem that it behooces the government to check it out," said Dr. Richard Hawks, a chemist with who is directing the institute's research. "There is a finite possibility that it comes into this country and a finite possibility that it has toxic results."

Paraquat has a toxicity rating of four to five on a scale of one to six or "very to extremely poisonous," according to toxicology textbooks. Hawks estimated that extrapolating from animal tests 45 ounces could be fatal to humans.

According to a memo to Percy from Daryl Dodson, a researcher on the Senate Governmental Affairs Permanent Subcommittee on Investigations the herbicide when ingested orally causes damage to the heart, lung, adrenal glancs and kidneys. There is no known antidotc, lung damage is permanent and death results from respiratory insufficiency.

Hawks said paraquat is not present in marijuana smoke, but added it is possible that paraquat residue could be transformed into other toxic chemicals by the hear of burning. But if marijuana containing paraquat is eaten in brownies or cookies for example, then the chemical does enter the body and could cause problems. Hawks said it would take a pound or more consumed in fairly short order to cause ill effects.

Because it takes days for the herbicide to kill the marijuana plants, it is possible that plants could be harvested and sold before paraquat does its job. If harvested early, Hawks said, the plant would "look yellowish and have burn holes or weird spots on the leaf."

Lee I. Degoloff, Bourne's deputy, said he, too is concerned about the chemical. But if the institute study shows its presence in this country and if it is determined that paraquat is harmful to marijuana consumers estimated at from 13 million to 15 million Americans. It is still not clear what can be done or what, if anything, the Carter administration might recommend.

"It raises some difficult policy questions." Dogoloff said in a telephone interview because we're still talking about a drug that is illegal.

State Department official Richard A. Dugstad of the office of international narcotics matters said, "We have done nothing to discourage the use of paraquat by the Mexican government."