The effects of feeding, throughout gestation, a diet deficient in protein, of pair-feeding, and of supplementing the deficient diet late in gestation on maternal body weight and weights of the products of conception were investigated in the rat. Protein deprivation resulted in net loss of maternal body weight, smaller and thinner placentas with decreased DNA content and placental labyrinth size, reduced uterine tissue weight, and smaller fetuses with retarded bone development. Pair-feeding caused a less severe reduction in maternal, uterine, placental, and fetal weights. Placentas from pair-fed dams had normal cell numbers with reduced cell size. Diet supplementation late in gestation resulted in increased net and total maternal body weight and uterine, placental, and fetal fetal weights, and an apparent partial recovery in fetal bone development. Placental cell size also increased significantly. The results suggest that placental and uterine development in protein-deficient dams is not the limiting factor in fetal development. Availability of protein may be the primary limiting factor, and energy deficit may play a secondary role.