To evaluate the role and mechanism of action of calcitriol on glucose-induced insulin secretion in uremia, 17 patients with severe chronic renal failure were studied. Glucose metabolism was investigated by the intravenous glucose tolerance test (IVGTT) before and after treatment for 21 days with 0.5 µg/day of calcitriol and 500 mg/day of calcium (C + Ca) (6 cases) or 0.5 µg/day of calcitriol alone (C) (11 cases). After these evaluations the patients on C + Ca were shifted to C and 6 patients on C were shifted to C + Ca, and IVGTT was repeated 21 days after the shift. For each test plasma glucose (G), immunoreactive insulin (IRI) and C-peptide (C-p) were measured at -30,0,2,5,15,30,45,60 min, and baseline plasma values of 1α,25(HO)2-vitamin D3, C-terminal parathyroid hormone (PTH-C), intact parathyroid hormone (PTH-I), calcitonin, and serum values of total and ionized calcium were dosed. Also, glucose constant decay (K-G), insulin response (IRI area), C-p production (C-p area), insulinogenic index (IGI) and insulin resistance index (RI) were calculated. A historical group of 21 healthy volunteers formed the normal controls. 1α,25(HO)2-vitamin D3 plasma levels in uremic patients before treatment were significantly lower than normal range. As compared to controls, uremic patients showed significantly lower K-G, IRI area and IGI values and significantly higher RI values. After treatment with C or C + Ca, the insulin response improved significantly at 2 and 5 min and G decrement was more marked at 30,45 and 60 min. K-G and 0-15 min IRI area, 0-15 min IGI and 0-15 min C-p area values increased significantly but did not reach the normal values. Serum calcium and plasma 1α,25(HO)2-vitamin D3 levels significantly increased after treatment but no correlations were found between changes in glucose metabolism parameters and changes in total or ionized serum calcium or changes in 1α,25(HO)2-vitamin D3 plasma levels. Serum calcium values significantly decreased after shift from C + Ca to C and significantly increased after shift from C to C + Ca, but glucose metabolism parameters showed no further changes. From these data we inferred that 1α,25(HO)2-vitamin D3 deficiency may contribute to the inhibition of glucose-induced insulin secretion in uremia. Calcitriol mainly influences the first phase of insulin release and its effect may be due to a direct action on the pancreatic