The relative proportions of prolactin cells, somatotrophs and gonadotrophs were determined in the adenohypophyses of 28 Merino ewes on selected days during the oestrous cycle. Plasma prolactin levels were measured in 16 of the animals at 3-hr intervals during the 24 hr before autopsy. From Days 1 to 5 of the cycle (Day 0=day of ovulation), plasma prolactin levels were 20-40 ng/ml and the prolactin cells were filled with granules. Plasma levels increased to 168 ng/ml between Days 6 and 9, and marked degranulation of the prolactin cells occurred. The greatest degranulation was found on Days 16 and 0, and was associated with a prolactin surge to a concentration of 610 ng/ml. The rise in plasma prolactin and intensive degranulation of prolactin cells at the time of ovulation and formation of the CL suggest that prolactin is important at this stage of the oestrous cycle.