were measured in eight adult anephric subjects. All were undergoing hemodialysis and three of them were receiving vitamin D, 50,000 or 100,000 U/d. Serum vitamin D was elevated in two of the patients given vitamin D and was abnormally low in the others. Mean serum 25-OHD was increased in patients given vitamin D (94.0±7.6 ng/ml) and was normal in the others (16.4±0.9 ng/ml, P < 0.001). Mean serum 24,25(OH)2D was normal in patients given vitamin D (1.38±0.27 ng/ml) and was low in the others (0.25±0.08 ng/ml, P < 0.001). Serum 24,25(OH)2D correlated significantly with serum 25-OHD (r = 0.848, P < 0.01). Mean serum la,25(OH)2D determined by receptor assay was 5.8±1.9 pg/ml in patients who were not given vitamin D and was 14.1±0.6 in those who were given vitamin D (P < 0.001). Serum la,25(OH)2D correlated significantly with serum 25-OHD (r = 0.911, P < 0.01). Mean serum la,25(OH)2D, measured by bioassay, was 8.3±1.9 pg/ml in patients who were not given vitamin D and was 15.9±2.4 pg/ml in those who were given vitamin D (P < 0.05). There was a significant correlation between the values for serum la,25(OH)2D obtained with the two methods (r = 0.728, P < 0.01). The results (a) provide evidence in man for extrarenal production of both 24,25(OH)2D and, by two independent assays, of la,25(OH)2D, and (b) indicate that serum

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