Daily blood samples from four mares were assayed for cortisol through a total of eight ovulatory cycles. Mean cortisol concentrations on days −14, −13, −10, −9 and −8 before ovulation (dioestrus) were greater than on days −5 to −1 (oestrus). The highest mean (±s.e.m.) value of cortisol occurred on day − 10 (260± 28 nmol/l) and the lowest on day −2 (142 ±14 nmol/l). A single episode on a day in late dioestrus characterized the maximum cortisol value per cycle for five of eight cycles. Extraction of plasma samples with petroleum ether or chromatography before assay, to eliminate interference from progesterone and its metabolites, did not alter the pattern of high dioestrous and low oestrous cortisol concentrations. Maximum follicular diameter at ovulation was negatively correlated with mean cortisol concentration for that cycle. These results indicate that in the mare the adrenals secrete cortisol more actively during dioestrus than during oestrus and suggest that a decline in cortisol values at oestrus may favour full follicular growth and ovulation.