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Adrenal cells were prepared from non-pregnant (anoestrous) sheep, from ewes at days 50, 100 and 130 of pregnancy and at term, and from animals at 1–5 days post partum. The ability of the cells to respond to adrenocorticotrophin (ACTH_1–24), -melanocyte-stimulating hormone (-MSH), or combinations of these peptides has been examined in vitro. There was a progressive rise in the basal output of cortisol during pregnancy and in the absence of adrenocorticotrophin the cortisol output from adrenal cells of late pregnant and post-partum sheep was significantly greater than that from the non-pregnant animals. Adrenocorticotrophin increased cortisol output by adrenal cells at all times tested. In anoestrous sheep the amount of ACTH required to produce half the maximum output of steroid (ED₅₀) was 8 pg/ml. The ED₅₀ increased in early pregnancy to 112 pg/ml and then fell to < 5 pg/ml between day 100 and term. At term both the stimulation ratio and the absolute increment in cortisol output elicited by a maximal concentration of ACTH were greater than at any other time tested in pregnant or non-pregnant sheep. Cortisol output during pregnancy was not increased by -MSH, although at term the stimulatory effect of ACTH_1–24 was partially antagonized by -MSH.

These results suggest that there may be an increase in the responsiveness of the maternal adrenal during pregnancy, although the factor(s) responsible remains unknown.