Growth hormone has been shown to possess stimulatory effects on various connective tissues. We observed that skin growth in male rat phosphoenolpyruvate carboxykinase-bovine growth hormone transgenic mice (serum growth hormone levels: 740-1940 ng per ml) is progressive with age, resulting in an "oversized coat" phenotype with a marked increase in absolute and relative skin weight and surface area, and in thickness of the dermis. Histologic changes include severe dermal fibrosis and replacement of subdermal adipose tissue by fibrous tissue. Apart from an increase in skin surface area, these changes were not noted in female transgenic mice, arguing for a specific interaction of growth hormone with male sex hormones. To clarify this point, 6 wk old male transgenic mice and control mice were castrated and compared with their noncastrated counterparts in parameters of skin growth at an age of 8 mo. The skin weight of castrated transgenic mice was smaller (p < 0.01) than that of intact transgenic mice both absolutely and relative to body weight. The relative skin weight of castrated transgenic mice was in the same range as in intact and castrated control mice. Absolute and relative skin area of castrated transgenic mice was greater (p < 0. 001 and p < 0.05) than in controls but lower than in intact transgenic mice (p < 0.001 and p < 0.05). When compared with control mice, intact transgenic mice displayed an increase (p < 0.01) in the thickness of dermis. In castrated transgenic mice the thickness of the dermis was in the same range as in control mice. Our findings demonstrate a specific interaction of growth hormone with male sex hormones resulting in a marked stimulation of skin growth.