In this presentation several of the reasons why androgenic hormones might be responsible for some of the effects of physical training and for some of the differences among individuals in athletic ability have been outlined. It seems apparent that minimal amounts of androgen are necessary to produce the normal differences between males and females in aggression, red blood cell production and bone growth, but it seems unlikely that any additional natural androgen, perhaps produced as a result of physical training, can be shown to enhance these characteristics after puberty. It seems more likely that changes in androgen levels or changes in sensitivity to androgens may be shown to be involved in the increases in muscle glycogen, protein synthesis, and lean body mass associated with chronic exercise. However, the data suggesting changes in androgen levels or androgen uptake with exercise are so meager and contradictory that no complete answer to any of these problems can yet be offered. Perhaps the best evidence that androgens are involved in athletic performance is that boys become more athletically proficient than girls at puberty, the time of the greatest changes in blood testosterone. The changes in muscle strength, body weight and lean body mass caused by anabolic steroid treatment are probably greater than many would hope and somwhat less than many think. There is apparently a wide range of individual responses to these drugs.