GH continues to be produced after the cessation of childhood growth and is the most abundant pituitary hormone in the adult pituitary. There is strong evidence that GH continues to exert significant biological effects on body metabolism in adult humans. The actions of GH may be direct or indirectly mediated by IGF-1. The direct actions of GH impart major effects on glucose, lipid and sodium homeostasis. GH administration causes hyperinsulinaemia and impairs the ability of insulin to suppress hepatic glucose production and to stimulate glucose uptake and oxidation. GH enhances fat utilisation by stimulating lipolysis and fat oxidation. The significance of these effects is reflected in the finding of increased adiposity in GH deficiency (GHD) and reduced fat mass in acromegaly. GH causes sodium retention which occurs in part through activation of the renin-angiotensin system. Extracellular water volume is diminished in GHD and increased in relation to the extent of GH excess in acromegaly. Lean body mass is reduced in GHD and restored by GH treatment. The anabolic actions of GH are mediated by IGF-1. Studies of whole body protein metabolism show that this occurs through the stimulation of protein synthesis, reduction in protein oxidation but not inhibition of protein breakdown. GH plays an important role in the regulation of substrate metabolism and body composition in man.