Experiments were designed to determine when the [M. eugenii] adrenal cortex begins to synthesize and secrete steroids. Production of both corticosteroids and androgens by adrenal tissue in vitro was about 5 ng/mg during the first 19 days of pouch life, but subsequently corticosteroid production rose, while androgen production fell. Plasma concentrations of corticosteroids and androgens were 2.8 .+-. 0.4 and 0.9 .+-. 0.3 ng/100 ml, respectively, in animals up to 39 days, and did not change significantly in the older groups (up to 139 days). In 2 fetal plasma samples (24 and 25.5 days, i.e., 1 or 2 days before birth) corticosteroid concentrations were 0.9 and 1.5 .mu.g/100 ml, respectively, and androgen concentrations were 0.8 and 0 .mu.g/100 ml. Adult values are 6-10 .mu.g/100 ml for corticosteroid and 0.5-1.5 .mu.g/100 ml for androgen. The view that changes in adrenocortical function may be associated with the onset of homoiothermy was not supported; the adrenal cortex of the tammar wallaby at, or even before birth, is probably already capable of synthesizing and secreting corticosteroids and androgens.