The blood clearance rate (BCR) of cortisol was measured in non-pregnant ewes and in pregnant ewes and their intact or bilaterally adrenalectomized fetuses. In pregnant sheep the placental transfer of cortisol in both directions was established. The BCR of cortisol in the non-pregnant sheep was 51·7±4·9 (s.e.m.)1/h (n = 36) or 1·151/h per kg body weight. This was lower than that in the pregnant ewe (97–143 days of gestation) of 76·9±4·21/h (n = 9) or 1·851/h per kg.
In the intact fetus the BCR was 8·2±0·261/h (n = 10) over the same period of gestation. The percentage of the maternal production rate of cortisol transferred to the fetus was 1·4±0·11% (n = 9) and the placental transfer from fetus to mother was 19·5±1·5% (n = 8). The BCR in pregnant ewes bearing bilaterally adrenalectomized fetuses was not significantly different from that of mothers of intact fetuses (58·4±7·71/h; n = 6). The BCR of adrenalectomized fetuses was 8·4±1·371/h (n = 8). The placental transfer of cortisol from mother to fetus was sufficient to account for all the cortisol measured in adrenalectomized fetuses and in intact fetuses of 100–121 days of gestation. However, it accounted for only 37% of the cortisol measured in fetuses of 122–135 days of gestation and 12% or less in fetuses older than 136 days of gestation.