Two monotocous and two ditocous ewes were infused at 124-130 days of gestation with a mixture of L-[U-14C]lactate and D-[6-3H]glucose via a maternal or umbilical catheter, separate days being used for each infusion. Plateaux-specific radioactivities of plasma lactate and glucose were used to derive a four-pool model describing the fluxes between the lactate and glucose pools of the infused conceptus and mother. The average turnover rate of lactate was 23.3 and 13.2 mg carbon min-1 in the conceptus and 32.0 and 44.0 mg carbon min-1 in the mother for monotocous and ditocous ewes, respectively. Glycolysis rates within both conceptus and mother in all sheep were high, accounting for about 80 and 60% of the respective rates of lactate turnover. The synthesis of glucose from lactate accounted for 13 and 31% of the glucose turnover in the mother in monotocous and ditocous ewes, respectively, but was insignificant in the conceptus. Glycolysis within the conceptus used only glucose which had entered and mixed with the conceptus glucose pool; there was no direct transfer of carbon from the maternal glucose pool to the conceptus lactate pool. This finding is an important validation for the use of tracer methods to determine glucose use within the whole conceptus rather than within fetal corporeal tissues alone.