W e measured plasma calcitonin concentrations in healthy volunteers (20 men, ages 23-45 yr, mean, 30 yr; 25 women, ages 21-46 yr, mean, 30 yr) with a radioimmunoassay capable of detecting 5 pg of calcitonin/500 μl incubation volume, or 25 pg/ml of unextracted plasma. All subjects had 4-h calcium infusion (15 mg Ca/kg), and 24 subjects had intravenous pentagastrin injection (0.5 μg/kg) on separate days. Men had higher basal plasma immunoreactive calcitonin concentrations than women (P < 0.001): mean, 49 pg/ml (range, <25-73) and 31 pg/ml (range, <25-51), respectively. 18 of the 20 men (90%) responded to induced hypercalcemia with increases in plasma immunoreactive calcitonin; only 14 of the 25 women (56%) responded. In men, the mean increase of plasma immunoreactive calcitonin±SE was 58±9 pg/ml, but for women was only 25±6 pg/ml. 8 of 10 men (80%) responded to pentagastrin with an increase of plasma immunoreactive calcitonin >30 pg/ml, compared with such a response in only 1 of 14 women (7%). These differences of plasma immunoreactive calcitonin responses between the sexes were statistically significant (calcium infusion, P < 0.02; pentagastrin, P < 0.001). The physiologic importance of these observations is unknown, but we speculate that a lifelong, relative deficiency of calcitonin in some women could play a role in age- and sex-related bone loss, particularly during the estrogen-deficient postmenopausal years.