The National Park Service is to call a halt next week to the use of the nation's most poplar weedkiller, 2,4-D, in its 325 parks and recreation areas.

The chemical, one of the constituents of the herbicide Agent Orange that was sprayed in Vietnam, is in common commercial use, and tons of it are used every year nationwide to kill weeds along highways, on golf courses and on millions of lawns. Figures were unavailable on how much the Park Service used nationally last year, but some 60 pounds were used to control dandelions on the Mall, the White House grounds and other park lands here.

The Environmental Protection Agency is studying the chemical, but officials there said evidence of its health effects is inconclusive so far and there are no plans to curtail its use.

Park Service Director Russell Dickenson confirmed that he had written a memorandum to all park superintendents ordering them to halt use of the herbicide immediately. Future use is to be allowed only if the superintendents convince their regional directors that no alternative methods are available to accomplish indispensable weed control.

A spokesman for the park service Duncan Morrow, said this was "nearly but not quite" a ban on 2,4-D, "but it will halt its use for now and in the long run . . . it will substantially reduce its use."

Morrow said the action followed pressure from environmental groups, many in the Washington area, which argued that the herbicide had been found to cause spontaneous abortions, bleeding of fetuses and birth defects.

"It's a matter of considerable controversy, and the evidence is inadequate to make any firm judgment," Morrow said. "Therefore [Dickenson] decided that it's better to err on the side of conservatism and not use a potentially dangerous chemical unless we can find that our fears are unfounded."

He said the action was "more a response to the uncertainty" than it was to any evidence of harm from 2,4-D. At Dow Chemical Corp., which is one of several manufacturers of 2,4-D, special projects consultant Wendell Mullison said the decision was distressing: "They're disregarding all the scientific information that's available on the subject." He said there had been "absolutely no problem" connected with 2,4-D in the 30 years it has been on the market.

He estimated that 25,000 to 35,000 tons of the chemical are sold yearly to farmers and other large-scale users under several manufacturers' brand names, including Weedone, Estron, Weed-Rhap and Brush-Rhap, and to homeowners as Weed-B-Gon and Formula 40.

Although it was one of the constituents of Agent Orange, 2,4-D was not the controversial part. That was 2,4,5-T, which was found to be contaminated with deadly dioxins and has subsequently been banned from most uses in the United States.

Hundreds of Vietnam veterans have filed suit and have sought compensation for a wide range of illnesses they claim resulted from their exposure to Agent Orange. The EPA is holding hearings on whether to ban all remaining uses of 2,4,5-T.

L. D. Blanchard of EPA's Pesticides Division said dioxins of the Agent Orange sort have so far never been detected in 2,4-D. While tests have shown that large doses of 2,4-D are toxic to animals, he said, small doses appear to have no effect. The agency has asked 2,4-D manufacturers for additional tests, but conclusive results are not expected for two years, Blanchard said.

An April fact sheet on the issue said EPA "believes the risks of several other pesticides are higher and better documented than those associated with 2,4-D." Blanchard said the EPA is eager to see any new evidence the park service may have. "If we receive anything that makes us believe suspension is warranted, we'll act quickly," he said. Erik Jansen of Friends of the Earth, one of those instrumental in persuading Dickenson of the need for his action, said existing literature "is already overwhelming" in providing evidence of spontaneous abortions and fetal bleeding. "There's absolutely no doubt it's a problem chemical," he said.