Rats were Cesarean-sectioned, ovariohysterectomized or ovariectomized on day 19 of gestation and the role of prolactin and corticosteroids in the initiation of lactation was studied by administering 2-bromo-.alpha.-ergocryptine (CB 154) and prolactin, and by using adrenalectomy and fetectomy. The concentrations of corticosteroids and prolactin in the plasma and the weight and lactose content of the inguinal mammary glands were determined 48 h after Cesarean section, and 24 h after ovariohysterectomy and ovariectomy. The ranges in concentrations of corticosteroids, prolactin and mammary lactose were 207-348 .mu.g/l plasma, 21.9-65.3 .mu.g/l plasma and 1.21-4.50 mg/g tissue, respectively, in the Cesarean-sectioned, ovariohysterectomized and ovariectomized rats. The administration of CB 154 after Cesarean section, ovariohysterectomy or ovariectomy decreased the concentration of prolactin to < 8.8 .mu.g/l. CB 154 significantly depressed the lactose content of the mammary tissue in Cesarean-sectioned and ovariohysterectomized rats (0.42 and 0.31 mg/g tissue, respectively), but no effect was observed in ovariectomized rats (1.39 mg/g tissue). The administration of ovine prolactin reversed the inhibitory effects of CB 154. In 2 rats which were both ovariectomized and fetectomized, CB 154 did not inhibit lactose accumulation in the mammary tissue (2.02 mg/g tissue). Adrenalectomy of ovariohysterectomized rats decreased corticosteroid concentrations from 348 to 14 .mu.g/l plasma and the lactose content of mammary tissue from 1.31 to 0.19 mg/g tissue. Adrenalectomy of ovariectomized rats decreased corticosteroid concentrations to 53 .mu.g/l; the lactose content of mammary tissue (1.82 mg/g tissue) remained within the range for ovariohysterectomized rats. Hormones produced in late pregnancy by the fetus and placenta can support lactogenesis in the rat in the absence of maternal prolactin and corticosteroids.