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An investigation was made of endocrine changes occurring during the life history of Phascogale calura, a small, arboreal, squirrel-like insectivorous marsupial. Following a 3-week mating period in July each year, all males disappear from the population and it is presumed that they die. During the mating period the plasma cortisol concentration in males increased to a greater extent than did the plasma corticosterone. During this period the plasma concentration of CBG decreased in the males but not in the females, which resulted in competition between the two glucocorticoids for binding sites and a 25-fold sex difference in free cortisol concentration. The plasma testosterone concentration in males reached its maximum value about the time of disappearance of the males and the plasma lacked high-affinity binding for androgens. The depression in CBG concentration was androgen dependent since castration of …