The Sertoli cells in the testes of Rattus fuscipes have been studied during the seasonal reproductive cycle of this Australian rodent using light and electron microscopy combined with the measurement of androgen binding protein (ABP) in cytosols from testis and caput epididymis. During the winter months when spermatogenesis was arrested and the testes were regressed, serum FSH, LH and testosterone levels were significantly lower than in the summer period, when the animals were sexually active. In the period of sexual quiescence, the seminferous tubule diameter was decreased and the epithelium consisted of Sertoli cells and spermatogonia. The height of the Sertoli cells was reduced and the nucleus was displaced toward the lumen. The nucleus was ovoid and decreased in size compared with the active state. Chromatin clumps were found adjacent to the nuclear membrane in contrast to the homogeneous nucleoplasm found in the Sertoli cell of the active testis. Diminished quantities of smooth endoplasmic reticulum and an accumulation of lipid droplets in the Sertoli cell cytoplasm also characterized the regressed state. Levels of ABP in cytosols prepared from testes and caput epididymides were significantly lower in the nonbreeding season, indicating reduced secretory function of the Sertoli cell.