Although age-related decline in plasma growth hormone (GH) levels is well documented, the possible role of GH in the control of aging is controversial. Overexpression of GH in transgenic mice is associated with reduced life expectancy and numerous symptoms of premature aging. Ames dwarf mice with hereditary GH, prolactin, and thyrotropin deficiency live much longer than their normal siblings. In contrast to these indications that GH may accelerate aging, some physiological changes in the elderly resemble symptoms of GH deficiency and can be corrected by GH replacement. It is suggested that these seemingly contradictory observations are related to the dose-response characteristics of GH action, and to negative correlation between body size and life expectancy within a species. Physiological mechanisms linking plasma GH levels and body size with aging remain to be identified.