Placental blood flow was measured with the aid of radioactive microspheres, in normal (N) and manifest diabetic (MD) rats, and related to fetal body growth and incidence of congenital malformations. The total blood flow in the placentae of the MD rats was decreased to about one-half of the normal flow on gestational days 20 and 22. The placentae of the MD offspring were enlarged, whereas the fetuses in this group were smaller than normal. Thus, the placental blood flowper placental weightwas drastically decreased in the MD fetuses on both days 20 and 22. In contrast, the placental blood flowper fetal weightwas not different in the N and MD groups on gestational day 20 whereas it was decreased in the MD offspring on gestational day 22. Placental blood flow in the malformed fetuses of the MD group did not differ significantly from that in the nonmalformed MD fetuses.