In spite of the importance of prolactin and adrenal corticosteroids in the normal control of the mammary gland (Cowie & Tindal, 1971) there has been no report of their simultaneous measurement in rat plasma during pregnancy and lactation. There is no agreement on the pattern of change in corticosteroid concentration at the time of lactogenesis and parturition (Gala & Westphal, 1965; Kuhn, 1969). Primiparous CFE strain rats, allotted randomly to groups on day 0 of pregnancy, were decapitated without prior disturbance within 1 min after removal from their cage, between 08.30 and 09.30 h. Plasma was stored at −20 °C, total body weight after bleeding and adrenal weight were recorded. The day on which a vaginal plug was found and the day of parturition were designated day 0 of pregnancy and lactation respectively. Litter size was adjusted to six on day 0 of lactation. Plasma prolactin concentration was assayed in