Sex differences exist in steroid and xenobiotic metabolism in the liver of a number of species. In the rat, the differences are regulated through the hypothalamo-pituitary axis. The previously postulated "feminizing factor" responsible for a female-type liver metabolism appears to be identical to growth hormone. The different effects of this peptide on hepatic metabolism in male and female rats may be related to the sexual dimorphism of the growth hormone secretory pattern; serum levels of growth hormone do not fluctuate as markedly in female as in male rats and may be simulated by administration of the hormone via osmotic minipumps, a procedure resulting in "feminization" of liver metabolism of male or hypophysectomized rats. This newly discovered system, the hypothalamo-pituitary-liver axis, represents a novel concept in endocrinology.