Involution of the thoracic thymus in two species of marsupial mouse, Antechinus swainsonii (Waterhouse) and Antechinus stuartii (Macleay) was shown to be unrelated to corticosteroid action and to be complete before puberty. A stress response in male marsupial mice is caused by an androgen related drop in the plasma corticosteroid binding globulin concentration which gives rise to an increase in the plasma free glucocorticoid concentration. The high concentrations of free glucocorticoids in the plasma just prior to the breeding season causes a rapid involution of the spleen and lymph nodes while the gut associated lymphoid tissues remain unaffected. The concentration of free glucocorticoids also rises in females, but it never attains the high concentrations observed in males. Nevertheless, the spleen and lymph nodes do involute to some extent in some females and the degree of involution appears to be related to the relative concentration of plasma free glucocorticoids. At the conclusion of the breeding season, there is a complete mortality in males of the population, due to a stress response in which the compromised immune system clearly plays a role.