1. Five well-nourished Scottish Blackface ewes carrying twin lambs were used. An umbilical artery and vein of each foetus in two sheep and one foetus in three sheep were catheterized at 109-112 days of gestation and both maternal jugular veins were catheterized 2 days before experiment. Two-day experiments were conducted between days 124 and 141 of gestation when [2-3H,U-14C]glucose was infused intravenously over 9 h into either the mother or one foetus, separate days being used for each infusion. 2. Plasma glucose specific radioactivities of the mother and infused foetus at plateau were used to determine the glucose flux rates within and between mother and foetus. The corresponding values for the uninfused foetus were calculated by using the relative weights of each twin foetus and were used to correct the maternal flux rates for the presence of the second foetus. 3. The net rate of glucose utilization by the foetus was 11.2 +/- 1.3 mg/min per kg of foetus (mean +/- S.E.M; n = 11). The corresponding rate in the mother was 1.1 +/- 0.1 mg/min per kg of live weight (n = 11). 4. The net rate of foetal uptake of glucose from the mother accounted for only 56 +/- 4% of the glucose used by the foetus. The remainder was provided by an input of 4.9 +/- 0.7 mg/min per kg of foetus to the foetal glucose pool via foetal gluconeogenesis. 5. The rate of glucose futile cycling in the foetus was high, equal to 95 +/- 19% of the net foetal glucose-utilization rate. 6. The rate of glucose utilization (mg/min per kg of foetus) decreased progressively over the observational period of 124-141 days of gestation. There was no increase in the rates of net foetal glucose uptake or of foetal gluconeogenesis to counter this decline.