Although basal serum LH and FSH levels have been shown to increase with age in men, there is evidence that aging may impair pituitary gonadotropin secretion to some extent. We have measured serum LH and FSH before and after the iv injection of 100 micrograms LRH in 69 healthy men, divided into 3 age groups: A (25-49 yr; n = 24), B (50-69 yr; n = 23), and C (70-89 yr; n = 22). We have previously demonstrated these men to have unaltered serum androgens and estrogens over the entire age range. Only log-transformed measures were analyzed in order to equalize variance among groups. Because we found increasing age to be associated with significant rises in basal levels of LH and FSH, and because the basal level affects the magnitude of response, we used repeated measures analysis of variance to compare LRH peak responses with basal gonadotropin levels. The presence of a significant interaction term would indicate an effect of age on response beyond that expected from the effect of basal level alone. The mean LRH responses (log peak--log basal) decreased, with significant interaction terms for both LH (P = 0.001) and FSH (P less than 0.0001) responses. This means that the function relating basal to peak gonadotropin concentrations changed significantly with age. Peak LH responses in the youngest group (A) occurred equally as often 15 and 30 min after LRH injection, but in the oldest group (C) there were no 15 min peaks, and 40% of the peak values occurred at 60 min (P less than 0.01). FSH peaks were equally distributed at 30, 60, and 120 min in all 3 groups. Total testicular volumes did not decrease significantly with age. We conclude that there is a small but statistically significant tendency for LH and FSH responses to LRH to be both diminished and delayed in healthy aging men. The mechanisms responsible for this apparent pituitary gonadotropic hypofunction remain to be clarified.