Ethanol pharmacokinetics were determined following oral ethanol, 0.5 gm per kg, in nine normal women and 10 normal men, and related to total body water measured by 'H-water dilution and body fat determined anthropometrically. Ethanol pharmacokinetics were similar in the females throughout the menstrual cycle. No variation was seen in mean peak blood ethanol concentration or elimination rate in the midfollicular (Days 8 to 10) and midluteal (Days 22 to 24) phases. Mean peak blood ethanol values were significantly higher in females (88 f 3 mg per 100 ml) than in males (75 f 4 mg per 100 ml) (p < 0.05), and the mean area under the ethanol concentration-time curve was significantly greater in females (241 f 12 mg.hr per 100 ml) than in males (177 f 11 mg. hr per 100 ml) (p < 0.001). There was no significant sex difference in mean ethanol elimination rates. The mean apparent volume of distribution of ethanol in female (0.59 f 0.02 liter per kg) was less than in males (0.73 f 0.02 liter per kg) (p < 0.001). Both apparent volume of distribution of ethanol and area under curve were significantly correlated with total body water suggesting that the sex differences in ethanol pharmacokinetics were due to sex differences in body water content. The sex differences in ethanol pharmacokinetics may partly explain reports of malefemale differences in the natural history of certain ethanol-related disorders.