SUMMARY Corticotrophin (ACTH), hydrocortisone or prednisolone trimethylacetate were administered daily to sows from 24 hr. before oestrus or from day 14 of the cycle until the end of oestrus. Treatment beginning on day 14 delayed the peak in urinary oestrogen excretion normally seen, postponed the onset of oestrus by 1–3 days and shortened the duration of oestrus. Treatment begun before oestrus did not appear to alter the pattern of oestrogen excretion but shortened the duration of oestrus. Administration of ACTH, but not that of corticosteroids, also interfered with ovulation.