The concentration of oestrone sulphate in peripheral plasma from postmenopausal women was investigated using a method which involved extraction of the conjugate, which was then hydrolysed with acid, and determination (by radioimmunoassay) of the purified oestrone fraction obtained. The concentration of unconjugated oestrone in the same plasma samples was also measured. Postmenopausal women had concentrations of oestrone sulphate in plasma (1.1 +/- 0.36 nmol/l, mean +/- SD, n = 39) similar to those found in women in the follicular phase of the menstrual cycle and less than those found in males (3.2 +/- 0.61 nmol/l, n = 21). The mean ratio of the concentration of oestrone sulphate to that of oestrone in plasma from postmenopausal women (7.9 +/- 3.3) was significantly lower (P less than 0.001, t test) than the mean ratio in men (19.8 +/- 3.8). Treatment with conjugated oestrogens, oestradiol in a cream, oestradiol valerate or ethinyl oestradiol, increased the concentration of oestrone sulphate in the peripheral circulation. In contrast, chronic corticosteroid therapy reduced the level of oestrone sulphate (0.5 +/- 0.11 nmol/l, n = 10) but this was partly restored (to 0.7 +/- 0.13 nmol/l) by concomitant oral dehydroepiandrosterone. Ingestion of piperazine oestrone sulphate (Harmogen, 1.5 mg) by three fasting postmenopausal women was followed 4 h later by oestrone sulphate concentrations five to ten times those found at midcycle.