We designed this study to determine whether the sexual behavior of male monkeys changes on a diurnal schedule that coincides with changes in the levels of hormones such as luteinizing hormone (LH), testosterone, estradiol, and cortisol, and whether these differences vary with age. Old (n = 8) and young (n = 8) males were bled and given sexual behavior tests eight times at 0900 and eight times at 2100. The old and young males did not differ in mean serum levels of testosterone, estradiol, cortisol, or LH. Testosterone and LH levels were lower at 0900 than at 2100 (p less than 0.01), and estradiol and cortisol levels were higher at 0900 than at 2100 (p less than 0.01). The young males had higher percentages of tests with erections, mounts, intromissions, and ejaculations than the old males (p less than 0.01). The rates of contacting, mounting, and intromitting were higher at 0900 than at 2100 (p less than 0.05). We failed to confirm previously obtained correlations between hormone levels and sexual performance, This failure led us to conclude that any significant correlation between sexual behavior and hormone levels must be regarded as tentative unless repeated in successive independent studies.