Studies of the pharmacokinetics of three contraceptive steroids, ethynyloestradiol, norethisterone and norethisterone oenanthate, in large groups of women are reviewed. Extremely wide variations were observed between women in the various calculated pharmacokinetic parameters. Studies carried out in small groups of subjects may give misleading results. It is difficult to ascribe any meaning and interpretation to most of the derived values but they do provide a means of comparing aspects of the metabolism of the various steroids. It is difficult to relate the pharmacokinetic values to the various biological actions of the steroids. However, the wide variations between subjects in the pharmacokinetic parameters must indicate equally wide variations in the biological activity of the steroids. Subjects in whom the bio-availability is low or where the rate of metabolism is rapid might be expected to respond differently, both in terms of efficacy and side-effects, from subjects with high values for bio-availability and a slow metabolism.