Treatment of pregnant guinea pigs with trichlorfon causes cerebellar hypoplasia in offspring. The most sensitive period for treatment is days 42-47 of gestation, which coincides with the rapid brain growth spurt and with the development of cerebellar granule cells. When rat granule cells were exposed in vitro to trichlorfon and dichlorvos for 24 hours they died, whereas trichloroethanol had no effect. When the cells were exposed to trichlorfon and dichlorvos for 3 hours, only dichlorvos was lethal indicating that the metabolite dichlorvos was more potent than trichlorfon itself. Cultured cerebellar granule cells were also found to be quite sensitive to other DNA-alkylating agents such as methylazoxymethanol and methylmethane sulphonate and to O6-benzylguanine; a potent and specific inhibitor of the DNA alkyltransferase involved in the repair of DNA alkylation damage. The organophosphorous compounds were also found to cause inhibition of the alkyltransferase and the lethal effects of the tested compounds on granule cell culture correlated well with the potency of inhibition. In a bacterial test system for monitoring alkylation effects on the DNA, dichlorvos was demonstrated to have a strong DNA alkylation effect. These results suggest that alkylation of DNA and inhibition of its repair can contribute to the brain hypoplasia observed after exposure to trichlorfon and dichlorvos during brain development.