Simultaneous measurements of placental transfer of 14C-alpha-aminoisobutyric acid (AIB), 3H-methyl glucose (MG) and placental blood flow were performed in anaesthetized guinea pigs at various gestational ages. The results were correlated with the growth of the conceptus. The placenta increased its weight at a maximal rate (g/day) between days 31 and 40 of gestation while maximal fetal increments in weight occurred between days 51 and 60. Total net transfer of AIB and MG per gram of placenta increased progressively during pregnancy while net transfer per gram of fetus decreased approaching term. Placental blood flow in the maternal side increased linearly near term. The quantities of AIB and MG transferred by the placenta per ml of blood was maximal between 51 and 60 days of gestation. These data suggest that a relatively smaller placenta in relationship with fetal size limits the quantity of nutrients transferred to the fetus near term, and reduces the rate of fetal growth.