The breeding season of S. macroura extends from June to February, and individual males (both wild- caught and laboratory-reared) are capable of breeding over extended periods during the breeding season, and for up to three seasons, in the laboratory. Gross and histological changes in the reproductive organs and endocrine changes in relation to reproductive activity have been investigated. Males do not appear to reach sexual maturity until the season following that in which they were born, although spermatorrhoea may commence in the season of birth. Testis and epididymis weight of these males, which commence spermatorrhoea late in the season, approximates that of sexually mature males early in the season but androgen levels and the weight of the accessory glands are low in all males except during the early months of the season. The age at which spermatorrhoea commences ranges from 141 to 350 days. The minimum scrota1 width at which it commences is 7.9 mm and the minimum body weight, 14.0 g. The onset of spermatorrhoea is not a function of age or season and in S. macroura should be used with caution as an indicator of impending sexual maturity. Maximum corticosteroid- binding capacity (MCBC) generally exceeded corticosteroid concentration and no androgen-related fall in MCBC was evident.