Tumor necrosis factor (TNF) is a cytokine that mediates many of the metabolic responses after endotoxemia, septicemia, and tissue injury. The effect of TNF on testicular function was determined in a series of studies in which rhTNF (0, 2, and 4 X 10(5) units/kg/24 hours) was administered by continuous infusion to male Wistar rats maintained on total parenteral nutrition adequate for growing rats. Testicular weight and histology, and plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone levels were determined at 1, 3, and 6 days. Testicular weight decreased within 24 hours and this was associated with a fall in plasma testosterone and increased LH and FSH levels. These changes persisted for 6 days, indicating a loss of testosterone-mediated negative feedback on gonadotropin release. Histologic examination demonstrated significant damage to the germ cells in the adluminal compartment of the seminiferous epithelium; extensive exfoliation of spermatocytes and spermatids occurred at day six. However the primary spermatogonia in the basal compartment were relatively spared. Damage to the seminiferous epithelium at earlier times was noted in some tubules. The decrease in testosterone concentration and increase in gonadotropin levels suggest that TNF interferes with Leydig cell function. Germ cell damage may be a direct effect of TNF on these cells or may occur through secondary mechanisms involving Leydig or Sertoli cell dysfunction.