The early effects of sex steroid therapy were assessed in 28 normal post-menopausal women, 18 treated with ethinyloestradiol and 10 with norethisterone. There was a reduction in the fasting urinary excretion of both calcium and hydroxyproline with both treatments, indicating reduced bone resorption. This was apparent after 1 week of therapy but became more marked after 3 weeks. These changes were not accompanied by any changes in plasma levels of calcitonin or parathyroid hormone. Patients receiving ethinyloestradiol showed a marked increase in plasma 1,25-dihydroxyvitamin D (1,25-(OH)2D) concentration but this was explicable entirely in terms of increased plasma levels of vitamin D binding protein. There was no change in the free plasma level of 1,25(OH)2D. Patients treated with norethisterone showed no increase in plasma concentrations of 1,25(OH)2D. We conclude that both ethinyloestradiol and norethisterone have a rapid and similar effect in reducing bone resorption. This is not mediated via the plasma levels of the calcium regulating hormones.