To identify possible age-dependent changes in the feedback relationship between the brain-pituitary and testes, we examined the minute-to-minute patterns of plasma luteinizing hormone (LH) and testosterone (T) in intact, young male rats and compared these profiles to those of old animals. Young (3 mo; n = 11) and old (22 mo; n = 12) Sprague-Dawley rats were fitted with indwelling venous catheters and between 24 and 48 h later, were bled without anesthesia, by remote sampling, at 10-min intervals for 8 h. Blood samples of 400 microliter were withdrawn, and an equivalent volume of a blood replacement mixture was infused after each sample. Plasma LH and T levels in each sample were measured by radioimmunoassay (RIA). Plasma T levels in old animals failed to show the transient oscillations observed in young animals. Mean plasma T levels were 50% lower in old compared to young animals (P less than 0.001). Plasma patterns of LH in old animals, like their younger counterparts, showed statistically significant episodic increases, whose apparent pulse frequency was inappropriately low for their circulating T level (although not statistically different from the young group). Pulse amplitude in the old animals was 66% lower in the old compared to the young group (P less than 0.015). We conclude that age-associated alterations in brain mechanisms governing LH secretion underline these endocrine changes.