1. Five monotocous and twoditocous Scottish Blackface ewes with indwelling catheters in an umbilical artery and vein of one foetus only and in both maternal jugular veins were used.
2. Experiments were conducted over a period of 2 d when [I4C]urea was infused intravenously over 9 h into either the mother or foetus, separate days being used for each infusion. Two series of experiments were completed, one in well-nourished and the other in undernourished sheep at 125–141) and 138–143 d of gestation respectively.
3. Plasma urea specific radioactivities of the mother and infused foetus at plateau were used to determine the urea flux-rates within and between mother and foetus. The mean rate of foetal urea production (Ffo) was 1.45 and 1.63 mg/min per kg foetus in well-nourished and undernourished ewes respectively. The corresponding rates of maternal urea production (Fmo) were 0.49 and 0.37 mg/min per kg live weight respectively and there was a close correlation between the rate of maternal urea disposal (Fom) and the dietary nitrogen intake.
4. The values of Ffo were used to calculate the maximum potential for foetal gluconeogenesis from deaminated amino acids. These calculations were compared with published information on the over-all rates of foetal gluconeo genesis in well-nourished ewes.
5. The foetal entry rate of urea expressed on a body-weight basis was high, approximately 8.5 times that of the mother, but it was a threefold overestimate of Ffo. The maternal entry rate was 1.3 times Fmo and the significance of this in relation to assessing differences in urea entry rates in pregnant and non-pregnant sheep is discussed.