SUMMARY Chronic foetal—maternal cortisol interrelationships were studied in five time-dated pregnant ewes. Serial blood samples were obtained from ewe and foetus simultaneously for several weeks before and including parturition. Plasma cortisol was measured by both fluorometry and isotopic competitive protein binding. The data indicate that maternal cortisol levels are slightly higher than those in the foetus during the latter part of gestation until approximately 1 week before parturition. During the last week of gestation, especially 4–5 days before delivery, a marked increase in foetal cortisol levels was noted. Administration of amino-glutethimide to the foetus 1 week before delivery did not alter cortisol levels in either foetus or ewe. The results suggest that parturition in sheep is a culmination of events initiated by active foetal adrenocortical secretion.