A study by Bell (A2) investigated the hypothesis that alteration of the vitamin D-endocrine system in blacks results from reduction in serum 25-hydroxyvitamin D and that the alteration is reversed by oral treatment with 25-hydroxyvitamin D3. The eight subjects (three men and five women) were studied while on no treatment (control) and after having been given 25-hydroxyvitamin D3 for 7 days (25-OHD3). The following are the urinary calcium (mg/d) determinations for the eight subjects under the two conditions.