AbstractDosages of methotrexate of roughly comparable embryotoxicity were determined to be 0.3 mg/kg on gestation day 10 in rats and 3.0 mg/kg/d on days 29 through 32 of gestation in rhesus monkeys. These times represented similar periods in development. This regimen in rats was moderately embryolethal, slightly teratogenic, and caused some intrauterine growth retardation; in monkeys it was mildly embryolethal, not teratogenic, and caused only transitory growth retardation.Methotrexate was largely cleared from maternal plasma of both species within 8 hours, and in spite of a higher administered dose and initial concentration in monkey, the rate of fall was faster than in rat. The fraction of total drug concentration remaining unbound in plasma was approximately 30‐40% during the first 4 hours after treatment in both species. Embryo concentrations, however, were strikingly different during the first 8 hours, ranging between 108 and 209 ng/g in monkey but between 3.4 and 7.7 ng/g in rat. A slow rate of fall in embryo concentrations in both species was estimated to be inversely proportioned to the rate of growth of embryos, which was in accord with the fact that the absolute amount per embryo was little changed in 24 hours.Thus, the degree and type of embryotoxicity was not closely correlated with the level or duration of concentration in the embryos: a small maternal dosage in rat produced moderate embryotoxicity and very low embryo concentrations; a large maternal dosage in monkey produced slight embryotoxicity despite high embryo concentration