Metabolites of vitamin D3 were measured in the circulation of four patients on chronic haemodialysis (three of whom were surgically anephric) before and during daily ingestion of 40 000 i.u. of cholecalciferol. Circulating 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] was measurable, but abnormally low before treatment; its circulating concentration rose in a substrate dependent manner when serum 25-hydroxyvitamin D3 (25-OHD3) increased, but the response was reduced when compared with the normal relationship. Serum 1,25-hydroxyvitamin D3 [1,25-(OH)2D3] and calcidiol lactone (25-OHD3-lactone) were consistently unmeasurable in sera from these patients before administration of cholecalciferol. However, when serum 25-OHD3 rose with treatment, 1,25-(OH)2D3 became detectable in the sera of three of the four patients and 25-OHD3-lactone could be measured in all of them. These data indicate that extrarenal sites of synthesis of 24,25-(OH)2D3, 25-OHD3-lactone and 1,25-(OH)2D3 exist in chronically dialysed patients but require large amounts of substrate to be significant.