Basal plasma levels of immunoreactive calcitonin (iCT), forearm bone mineral content (BMC) as measured by 125I photon absorptiometry and 24-hour urinary hydroxyproline/creatinine ratio (OHPr/Cr) were determined in 32 healthy women (13 pre-menopausal, aged 40 to 54 years, and 19 post-menopausal, aged 41 to 54 years). The basal plasma levels of iCT were significantly higher in the pre-menopausal group (mean value 96 vs 54 pg/ml, P less than 0.025). The BMC value of the radius was also significantly greater in the same group (mean +/- SEM 656 +/- 13 vs. 620 +/- 9 mg/cm2, P less than 0.05), while the urinary OHPr/Cr ratio was higher in the post-menopausal group (29.9 +/- 1.5 vs. 38.7 +/- 2.7 mg/g, P less than 0.02). These results suggest that basal plasma levels of iCT decrease after the menopause and support the hypothesis that a deficiency of CT could be involved in the pathogenesis of post-menopausal bone loss. Similar results were obtained in 25 uremic women on maintenance hemodialysis (9 pre-menopausal and 16 post-menopausal) aged 30 to 65 yrs.: both basal iCT levels and BMC values were significantly higher in the pre-menopausal group.