The aim of this study was to investigate the effect of daily oral administration of calcitriol on calcium metabolism in Japanese postmenopausal women. For this purpose, we administered 0.5 μg of daily calcitriol to 18 Japanese postmenopausal women for up to 24 weeks. During the first 28 days, daily administration of 0.5 μg of oral calcitriol increased fasting serum 1,25(OH) ₂D levels significantly in 9 women (Group B) (p<0.005), while no significant change was seen in another 9 women without calcitriol administration (Group A). The first 28-day calcitriol supplement increased fasting urinary calcium excretion (urinary Ca/Cr) from 0.133 ± 0.072 to 0.171 ± 0.089 (p<0.05) and fractional excretion of calcium (FECa) without changing serum Ca²⁺. Urinary NTx/Cr excretion, an index of bone resorption, decreased significantly from 64.8 ± 24.5 to 50.3 ± 27.2 nMBCE/mMCr in Group B. Following the 28-day control period, 0.5 μg of oral calcitriol was also administered to women in Group A for another 20 weeks. At the end of the 24-week investigation period, the effects of oral calcitriol on urinary calcium excretion and bone resorption were still significant in both Group A and B. A positive correlation was found between urinary Ca/Cr and NTx/Cr excretion at the start (r = 0.657, p<0.05), but this correlation was lost by calcitriol treatment (r = 0.135). These results indicated that calcitriol supplement was effective in suppressing bone resorption in postmenopausal women, and that an increased fasting urinary calcium excretion due to calcitriol supplement was predominantly caused by increased intestinal calcium absorption in these women.