Tumor necrosis factor (TNF) has been implicated as a mediator of many diseases associated with alterations in hypothalamic-pituitary-testicular (HPT) function. To assess the effects of TNF on the HPT axis, we performed a saline-controlled cross-over study in six healthy men, sequentially measuring serum concentrations of gonadotropins, testosterone, and sex hormone-binding globulin (SHBG) after a bolus intravenous injection of recombinant human TNF (50 micrograms/m2). TNF induced an early and transient increase in serum luteinizing hormone (LH) levels from 6.0 +/- 1.0 to a maximum of 8.0 +/- 1.0 U/L after 30 minutes (P < .005), whereas the concentrations of follicle-stimulating hormone (FSH) remained unchanged. The increase in LH concentrations was followed by a transient decrease in serum testosterone levels from 18.2 +/- 0.3 to 9.1 +/- 1.2 nmol/L after 4 hours (P < .0001). Remarkably, LH levels had returned to control values when the testosterone level reached its nadir. SHBG levels were not affected by TNF. Our results suggest that TNF affects the HPT axis at multiple levels and may be involved either directly or indirectly in the decrease in circulating testosterone concentrations in systemic illnesses.