Plasma levels of levonorgestrel and ethinyl oestradiol were estimated in six women who took part in a double blind cross-over trial of hormone replacement therapy. Over a 12 mth period, each woman received each of the following oral drugs for 3 mth: ethinyl oestradiol 50 microgram, levonorgestrel 250 microgram, combination of ethinyl oestradiol 50 microgram and levonorgestrel 250 microgram (Nordiol), and placebo. Plasma samples were collected at 2, 8 and 26 h after morning medication each month. Plasma levels of norgestrel were increased when ethinyl oestradiol was added in the form of the combined preparation "Nordiol". The addition of ethinyl oestradiol to norgestrel therapy also resulted in significant differences between patients in the plasma norgestrel levels. There was approximately a three-fold difference between individuals in the plasma levels of ethinyl oestradiol achieved on the same fixed dose. No simple relationship was evident between plasma levels of norgestrel or ethinyl oestradiol and any of the clinical features measured.