A University of Idaho scientist has found that immature female mice exposed to the insecticide methoxychlor develop irregularities in their reproductive systems.

Methoxychlor is widely used in this country for pest control as a substitute for DDT. It is sold under the brand name "Marlate" and is labeled for application to a wide variety of vegetable crops, young fruit trees, forage and feed crops, ornamental plants and use in grain storage bins.It also is approved to control insect pests on livestock, although there are some limitations on its use with dairy cattle.

Victor Eroschenko, professor of zoology and veterinary medicine at the University of Idaho, said he selected newborn mice for his study because their undeveloped reproductive tracts are sensitive to estrogen. He and some other scientists are concerned about the effects of methoxychlor on mammalian reproduction because it works very much like the female hormone estrogen.

He said mice showed abnormalities in their reproductive systems soon after treatment and at six months had serious problems.

Officials said one reason for the pesticide's widespread use is its low toxicity to humans. The ingestion of about one pound would be needed to be fatal to a 180-pound male.