The aminonucleoside of puromycin (PA) was administered to pregnant rats during early, mid- and late pregnancy. Abnormal nephrogenic development occurred in 20% of newborn rats of mothers treated with PA on the 8th, 9th and 10th days of pregnancy, a recognized critical period in the embryogenesis of the rat kidney. This resulted in fewer glomeruli and poor separation of cortex and medulla. Nevertheless, functional capacity appeared well preserved.