Two and one-half of mgAg 2,4-diamino-5-(3''4''-dichlorophenyl)-6-methylpyrimidine was given to groups of pregnant rats. Doses of 2.5 mg/kg given prior to implantation induced 40-60% of fetal death with stunting and malformations in the survivors. The same doses given at the time of implantation induced 90% or more fetal death and resorption. Larger doses of 5 and 7 mg/kg toxic to the mothers destroyed all litters. Leucovorin had a strong inhibiting effect on the action of the drug. 2,4-diamino-5-chlorophenyl-6-ethylpryrimidine was less effective in fetus and litter destruction. Doses of 12.5 mg/kg at the time of implantation affected 30-60% of all fetuses while toxic to the mothers. Ten small doses, varying from .3 to 1.0 mg/kg given during the first half of the gestation period caused an increased number of fetal resportions and malformations. Pteroylglutamic acid could not entirely prevent the effect of the drug.