The effects of intravenous 1,25 dihydroxycholecalciferol [(OH)2D3] on glucose tolerance and insulin secretion were studied in eleven uremic patients on regular hemodialysis and compared with eleven healthy controls. Intravenous glucose tolerance tests (IVGTT) were used to assess glucose tolerance, and the hyperglycemic clamp technique was used to quantitate endogenous insulin secretion. Three days after they had discontinued oral 1,25(OH)2D3, the dialysis patients were then studied with (+D) and without (-D) a single intravenous dose of 1,25(OH)2D3 at 2 micrograms/m2, given two hours before the IVGTT or clamp studies. During the -D studies, the uremic patients were glucose intolerant but not hyperinsulinemic. Intravenous 1,25(OH)2D3 in dialysis patients increased glucose uptake (K values) during IVGTT by 38% (P less than 0.02) and increased early component of insulin secretion during hyperglycemic clamps by 48% (P less than 0.01) and the late component by 32% (P less than 0.01). After intravenous 1,25(OH)2D3, the dialysis patients became hyperinsulinemic and regained glucose tolerance. Intravenous 1,25(OH)2D3 did not change the K values during IVGTT nor the insulin secretion during hyperglycemic clamps in the control subjects. During the -D studies, serum concentrations of 1,25(OH)2D3 were significantly lower in uremic patients compared with controls. Serum 1,25(OH)2D3 during the +D studies increased to supraphysiological levels in both uremic patients and controls. Serum concentrations of intact parathyroid hormone, total and ionized calcium, magnesium, potassium, urea nitrogen and creatinine were not different between the +D and -D studies in neither the uremic patients nor the controls. These results suggest that 1,25(OH)2D3 deficiency, independent of parathyroid hormone and calcium, may contribute to the abnormalities in glucose tolerance and insulin secretion in dialysis patients.