The rate of placental transfer of glucose and α-amino isobutyric acid (AIB) was studied at 14, 17, 18, 19, 20 and 21 days of gestation in normal rats and at day 22 and 23 (prolonged gestation) in rats with uterine ligation. Transfer of both substances increases linearly from day 14 to day 19, levels off between days 19 and 20 and drops to 75 per cent of day 20 levels at day 21. There is a 90 per cent increase in AIB transfer between days 14 and 20 compared with a 28 per cent increase in glucose transfer during the same period. Thus, although the developmental pattern was similar the capacity of the placenta to transfer amino acids develops later than glucose. Prolonged gestation markedly reduces the capacity to transfer AIB but has little effect on the placenta ability to take up this substance. This was demonstrated in experiments showing little if any change in uptake of AIB at days 22 and 23 compared to day 21. By contrast, AIB transfer was 30 per cent less than day 21 and 55 per cent less than day 20. Thus, there are marked developmental differences in the maturation of the transfer systems for glucose and amino acids. Prolonged gestation reduces transfer of both substances.