Placental transport and fetal utilization of leucine were studied at 130 days of gestation in six control ewes and in seven ewes in which intrauterine growth retardation (IUGR) had been induced by exposure to heat stress. Leucine fluxes were measured during simultaneous intravenous infusion of L-[1-13C]leucine into the mother and L-[1-14C] leucine into the fetus. In the IUGR group, the following leucine fluxes, expressed as micromol/min/kg fetus, were reduced compared with control: net uterine uptake (3.44 vs. 8.56, P<0.01), uteroplacental utilization (0.0 vs. 4.7, P<0.01), fetal disposal rate (6.4 vs. 8.9, P<0.001), flux from placenta to fetus (5.0 vs. 7.1, P<0.01), direct transport from mother to fetus (1.6 vs. 3.4, P<0.01), flux from fetus to placenta (1.5 vs. 3.2, P<0.001), and oxidation of fetal leucine by fetus plus placenta (2.1 vs. 3.2, P<0.02). Uterine uptake, uteroplacental utilization, and direct transport were also significantly reduced per gram placenta. We conclude that maternal leucine flux into the IUGR placenta is markedly reduced. Most of the reduced flux is routed into fetal metabolism via a decrease in placental leucine utilization and a decrease in the leucine flux from fetus to placenta.