SUMMARY: The concentrations of lactose and progesterone were determined in mammary secretion and blood plasma, respectively, collected from ten ewes at daily intervals from 30 days before to 10 days after parturition. An initial increase in the concentration of lactose in the mammary secretion occurred as early as 30 days before parturition and was not related to changes in the concentration of progesterone in the peripheral plasma. On the other hand, there was a rapid fall in the concentration of progesterone in the peripheral plasma from 1 to 4 days before parturition and this fall was closely related to a second rapid increase in the concentration of lactose in the mammary secretion. The foetuses were removed from an additional 12 ewes by Caesarean section at 144 days of gestation; three ewes received no hormone therapy; three ewes received 62·5 mg progesterone/12 h for 5 days; three ewes received 2·0 mg oestradiol at 12, 24 and 36 h respectively after surgery; three ewes received both progesterone and oestrogen therapy. Whereas progesterone inhibited the increase in milk yield and lactose concentration after Caesarean section, progesterone and oestrogen in combination inhibited only the increase in milk yield. These results suggest that progesterone withdrawal initiates lactogenesis in the ewe.