Very few investigations in man have been specifically devoted to the influence of sex on drug kinetics. Although the potential role of various factors such as the phase of the menstrual cycle, hormonal steroid contraceptive use, menopause or andropause is virtually never taken into account, sex-linked differences in drug kinetics do exist for a limited number of substances such as certain antibiotics, a few tricyclic antidepressants, lithium and aspirin. These sex-linked differences are relatively minor from a quantitative standpoint and appear to be due chiefly to differences in body composition and activity of circulating enzymes and hormones. On the basis of present knowledge, in general these differences in drug kinetics do not warrant modifying dosage as a function of sex.