Summarydaily subcutaneous injection of 0.2 ml of the vitamin-DzTo test the effects of chlorothiazide on vitamin-D2-induced hypercalciuria, we carried out 17 metabolic studies lasting 12 days each in adult Sprague-Dawley male rats. Three groups were studied: (A) control rats receiving only the vitamin-D2 vehicle; (B) vitamin-D2-treated rats receiving 50 IU/day; and (C) rats treated in the same manner as group B with the addition of chlorothiazide 20 mg/day for the last 6 days of the study. Urine was collected during the last 3 days, and a blood sample was obtained at the end of each study period. Analysis of the data showed that there were no significant differences between the groups in changes of serum calcium concentration (A, 6.1 + 0.1 mg/dl; B, 6.1 f 0.2 mg/dl; C, 6.0 f 0.2 mg/dl), serum creatinine concentration (A, 0.5 + 0.07 mg/dl; B, 0.52 f 0.08 mg/dl; C, 0.48 k 0.04 mg/dl), and creatinine clearance (A, 4.8 f 0.7 ml/min/kg; B, 5.2 + 1.2 ml/min/kg; C, 4.9 f 0.5 ml/min/kg). The administration of vitamin-D2 significantly increased the urinary calcium excretion from 6.7 f 1.0 mg/kg/ day to 19.5 f 9.7 mg/kg/day ( p < 0.02), but the calciuria was inhibited in group C rats by the addition of chlorothiazide, which restored urinary calcium excretion to 6.8 + 2.5 mg/kg/day ( p < confirmed these effects of vitamin-D2 and chlorothiazide on urine calcium excretion. We conclude that in rats conventional doses of vitamin-D2 consistently induce marked hypercalciuria, even witbout hypercalcemia, and that this hypercalciuria can be effectively prevented by chlorothiazide. vehicle only; Group B, the vitamin-Dz group was composed of six rats that were injected daily subcutaneously with 50 IU of vitamin-DP (Drisdol, Winthrop Laboratories, New York, NY); and Group C, the vitamin-Dz and chlorothiazide group included six rats that were treated in the same manner as those in Group B but additionally with chlorothiazide 20 mg/day injected intraperitoneally (Diuril, Merck Sharp and Dohme, West Point, PA) for the last six days. During the last 3 days of the study all urine was collected, mixed, and measured. At the end of the study a blood sample was obtained from each rat by aortic puncture.0The urine was analyzed for calcium, creatinine, and sodium, and the serum for calcium and creatinine. Calcium was determined by atomic absorption spectrophotometry, nonchromogen creatinine, and sodium by standard methods. Student's t test was used to determine the significance of the difference between the groups.