The concentration of acid soluble carnitine was determined in several body tissues and fluids in rats under various conditions of sex steroid regulation. Intact female rats had significantly greater liver carnitine concentrations and urinary excretion rates, and lower blood plasma and heart carnitine concentrations than intact male rats. Ovariectomy increased blood plasma carnitine concentrations (P less than 0.01) and the excretion of carnitine in the urine (P less than 0.05). The administration of either estradiol or testosterone to ovariectomized rats did not alter blood plasma concentrations or urinary excretion of carnitine. Orchidectomized rats had similar blood plasma carnitine concentrations when compared to intact males but excreted significantly (P less than 0.01) greater quantities of carnitine in their urine. Administration of testosterone to orchidectomized rats reduced (P less than 0.01), whereas estradiol stimulated (P less than 0.05) the excretion rate of carnitine in the urine; however, blood plasma carnitine concentrations were not affected by these hormones. These data suggest that a major site for modulation of body carnitine concentration in the male resides in the control of kidney excretion by androgens. Liver, heart and skeletal muscle carnitine concentrations were not altered by the administration of either estradiol or testosterone to orchidectomized or ovariectomized rats.