Bone disease may occur in disorders associated with chronic metabolic acidosis. This has been attributed, in part, to reduced production of 1,25(OH)2D3. Although metabolic acidosis in the vitamin D deficient animal has been associated with a reduction in the conversion of radiolabeled 25(OH)D3 to 1,25(OH)2D3, studies in D-replete humans have revealed no effect of acidosis on 1,25(OH)2D3 metabolism. To examine this issue further, we measured serum 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D levels in six healthy subjects before and after 9 days of metabolic acidosis induced by the ingestion of ammonium chloride. In four subjects, we measured the increment in serum levels of 1,25(OH)2D in response to the infusion of parathyroid extract both during control and acidosis. Serum levels of 1,25(OH)2D, 13.6 +/- 1.3 and 14.3 +/- 0.9 pg/ml, in control and acidosis, respectively, were not different. The serum 1,25(OH)2D levels in control and acidosis rose to a similar degree with the infusion of PTE. These data provide strong evidence that metabolic acidosis does not have a substantial impact on the synthesis of 1,25(OH)2D3 metabolism in vitamin D-replete humans.