Placental transport and fetoplacental utilization of threonine (Thr) were compared at 130 +/- 1 days gestational age between seven control ewes (C) and six ewes in which intrauterine growth restriction (IUGR) had been induced by exposure to high ambient temperature from 33 +/- 1 to 112 +/- 2 days of gestation. The fluxes were measured using simultaneous intravenous infusions of L-[1-13C]Thr into the mother and L-[U-14C]Thr into the fetus. The IUGR group had less fetal weight (1.27 +/- 0.14 vs. 3.10 +/- 0.10 kg, P < 0.01) and placental weight (120 +/- 17 vs. 295 +/- 14 g, P < 0.01) than the C group. The direct flux of maternal Thr into the fetal systemic circulation was less in the IUGR fetuses, both relative to fetal weight (1.40 +/- 0.19 vs. 2.19 +/- 0.18 mumol.min-1.kg fetus-1, P = 0.0107) and placental weight (1.5 +/- 0.2 vs. 2.3 +/- 0.2 mumol.min-1.100 g placenta-1, P = 0.0187). In both groups, there was excretion of CO2 produced from fetal Thr. The rate of CO2 production from fetal plasma Thr carbon by fetus plus placenta was reduced in the IUGR group (1.50 +/- 0.23 vs. 2.86 +/- 0.32 mumol.min-1.kg fetus-1, P = 0.0065). We conclude that the flux of maternal Thr into the IUGR fetus is markedly reduced because of a reduction in placental mass and because of a weight-specific reduction in Thr placental transport. The reduced flux is routed into fetal Thr accretion via a decrease in fetal Thr oxidation.