There is considerable interest in the relationship between testosterone and sexual behavior in men, but the few available data bearing on this issue are inconclusive. This study is an attempt to clarify the relationships among various components of sexuality and serum testosterone concentration through examination of these variables in a large homogenous group of men. Serum testosterone concentration was determined in two blood samples taken 1 week apart from 101 young adult men. Subjects reported their frequencies of various sexual activities and level of sexual interest. Testosterone concentrations in the two blood samples were highly correlated, indicating considerable intraindividual consistency. There was considerable variance among subjects in both mean testosterone and reported sexual activity. Testosterone concentration did not correlate with the sexual activity and interest variables. These results provide evidence that differences among men in circulating testosterone concentration within the normal range do not account for differences in sexual activity and interest. It is also unlikely that variations in sexual activity account for differences in testosterone concentration.