It is clear that the anthropometric ramifications, especially with respect to muscle mass, of the metabolic actions of GH and IGF-I treatment in intact and GH-deficient adults require further study. At present, it appears that daily GH or IGF-I treatment modestly increases nitrogen retention in most normal adults, probably by separate but permissive mechanisms, but only for a short period of time (approximately 1 month). During prolonged GH administration, resistance to the anabolic actions of GH seems to occur, and optimizing the anabolic effects of GH or IGF-I treatment will require a better understanding of the interactions among GH, GHBP, IGF-I production, IGFBPs, the GH dose regimen, and other unidentified regulatory factors. On the basis of the similar increases in muscle protein synthesis, muscle cross-sectional area, and muscle strength observed in placebo and GH-treated exercising young adults, it is doubtful that the nitrogen retention associated with daily GH treatment results in an increase in contractile protein, improved muscle function, strength and athletic performance. Even in catabolic or GH-deficient populations, GH treatment provides only modest increments in nitrogen retention, muscle size, strength, and exercise capacity. Further, the side effects of GH treatment (water retention, carpal tunnel compression, insulin resistance) would be a detriment, rather than an aid, to athletic performance. In addition, whether prolonged (> 6 months) GH treatment alone or in combination with other agents used by athletes (e.g., anabolic steroids, beta-agonists) is associated with other adverse side effects (e.g., cancer, diabetes) has not been evaluated. Therefore, health professionals should continue to discourage the use of GH by exercise enthusiasts.