Ontogeny of fetal hepatic and placental growth and in vitro oxygen consumption (VO2) was investigated in fetal lambs at 75, 100, and 136 days postconception. Fetal hepatic relative weight and placental absolute and relative weights declined during this period. Oxygen consumption per gram dry weight of fetal liver and maternal placenta declined between mid and late gestation while fetal placental VO2 was unchanged. Estimated VO2 of the whole placenta did not change while the estimated total hepatic VO2 increased more than threefold between 75 and 136 days. Total hepatic VO2 was highly correlated with total placental VO2 at 136 days (r = 0.84). The results suggest that the placenta reaches its maximum growth and metabolic capacity before 100 days and possibly at or before midgestation. Changes in hepatic weight-specific total VO2, in addition to the declining relative size of the fetal liver, must contribute to the progressive decline in metabolic rate of the whole fetus during the second half of pregnancy. Correlations between placental and fetal liver weights and metabolic rates suggest the possibility of placental regulation of fetal hepatic growth and metabolism.