The protective action of estrogens on bone mass may be mediated by an increase in calcitonin (CT) secretion. We reevaluated this hypothesis using a method for measuring CT in extracts of serum that allows sensitive specific measurement of CT monomer. We studied seven healthy postmenopausal women before and on the 7th and 28th days of each of three 4-week treatment periods: estrogen (estradiol valerate; 2 mg/day), calcium supplement (1500 mg/day), and estrogen plus calcium; the three cycles were separated by intervals of 4 weeks. Serum extractable CT (exCT) levels were measured before and after a short calcium stimulation test (2 mg Ca/kg in 5 min) to assess the C-cell secretory response on each day. Estrogen had the expected biological effects, decreasing (P less than 0.05) serum gonadotropin concentrations and fasting or 24-h urinary calcium excretion. The calcium supplement caused a significant increase in 24-h urinary calcium excretion. However, there was no increase in basal or stimulated serum exCT levels during any of the three cycles. On the contrary, basal serum exCT concentrations decreased slightly but significantly during estrogen treatment from 1.9 +/- 0.5 (+/- SE) to 1.5 +/- 0.4 ng/L on day 7 and 1.2 +/- 0.2 ng/L on day 28 (P less than 0.05). This decrease in basal exCT levels did not occur during the combined estrogen and calcium administration period, probably because the slight decrease in serum calcium induced by estrogen did not occur during combined estrogen and calcium administration. In summary, estrogens do not stimulate CT secretion; variations in serum exCT levels appear to be related to the changes in bone metabolism induced by estrogens.