Male Antechinus stuartii were collected from sclerophyll forests in Victoria at regular intervals from February to August. Spermatogenic function was assessed by means of light microscopy of testicular tissues fixed and embedded in epoxy plastic, from which a quantitative analysis of spermatogenesis was determined. Testis cytosols were prepared for assay of androgen-binding protein (ABP) and plasma was collected for androgen assay. Germ cell maturation proceeded normally until May, when failure of spermatogenesis was reflected by depletion of spermatogonia and early primary spermatocytes. However, germ cells of more advanced maturational stages were able to complete the spermatogenic process, yielding mature sperm first observed in the testis late in June. Failure of the seminiferous epithelium to replenish the numbers of early germ cells resulted in progressive depletion of germ cells in later months, leading in August to collapse of the seminiferous tubules which then contained only Sertoli cells and spermatogonia. Plasma androgens exhibited progressive elevation, reaching a peak in July and August, but testicular ABP was always undetectable. The findings suggest that the inability of the testis to maintain spermatogenesis results from intrinsic changes to testicular function exerted at the level of the spermatogonial population.