Five groups of pregnant Wistar rats were injected intraperitoneally with a single dose of semicarbazide (SC) on day 5, 7, 10, 13, or 15 of gestation. The lots in each group received either 50, 75, 100 or 150 mg/kg SC. A sixth group received 17 mg/kg of the drug during the entire course of pregnancy. SC produced a significantly smaller number of live fetuses with respect to controls. This toxic effect after injection at days 7 and 10 of gestation was highest for all single doses. The mean of fetal weight decreased with respect to controls with almost all of the SC treatment studied. The number of implantations and live fetuses was significantly decreased in the rats receiving the continuous treatment. Most abnormalities in the 21-day-old fetuses were found in the brain, kidney, intestines and liver; skeletal anomalies were seen in the skull, sternum and ribs. SC also produced high postnatal mortality rates during the first month of life with the single doses as well as with the continuous treatment. Thus, SC produced signs of toxicity and/or teratogenic effects in rats with all doses administered.