Fasting serum calcitonin levels were measured in 54 postmenopausal women who had for 10 years been taking part in a double blind trial to assess the effect of the synthetic oestrogen, mestranol, on postmenopausal bone loss. There were no differences in calcitonin levels between mestranol treated and placebo groups. Fifteen of the women were challenged with a calcium infusion to measure the secretory reserve of calcitonin. Oestrogen treatment did not increase the calcitonin response to calcium infusion. The three patients who exhibited the greatest responses were placebo treated. Bone density was measured by gamma-ray absorptiometry over the ten year period and the annual rate of change of bone density calculated. No correlation could be found between basal calcitonin level or calcitonin reserve and change in bone density. Our results indicate that postmenopausal osteoporosis is not caused by a deficiency of calcitonin and that the action of oestrogen therapy to prevent bone loss does not involve calcitonin.