Circulating vitamin D3 metabolites were measured in 31 adult patients with chronic renal failure and 31 adults between 3 and 30 months after renal transplantation. No subject excreted over 1 g urinary protein daily nor received vitamin D or its metabolites. There was a positive correlation between 1,25(OH)2D3 and GFR between 15 and 90 ml/min in both chronic renal failure (r = 0.60, P less than 0.001) and transplant subjects (r = 0.49, P less than 0.01) and between 1,25(OH)2D3 and 25(OH)D3 after transplant (r = 0.69, P less than 0.001), but not in chronic renal failure (r = 0.22, P = ns). There was a weak inverse correlation between 1,25(OH)2D3 and serum phosphate in chronic renal failure (r = 0.36, P less than 0.05) but not post transplant (r = 0.03, P = ns). Compared with 1,25(OH)2D3 concentrations in 16 normal subjects (mean +/- SEM: 39.5 +/- 1.9 pg/ml), chronic renal failure subjects with mild renal impairment (GFR 45-90 ml/min, mean: 61.5 +/- 3.3 ml/min, n = 17) had reduced 1,25(OH)2D3 (28.9 +/- 2.7 pg/ml, P less than 0.01). In transplant subjects with mild impairment (GFR 45-90 ml/min, mean: 61.4 +/- 3.7), 1,25(OH)2D3 was positively (r = 0.79, P less than 0.001) and iPTH inversely correlated (r = 0.51, P less than 0.05) with 25(OH)D3. In each of nine such subjects studied, seasonal variations in 1,25(OH)2D3 (P less than 0.001) and PTH (P less than 0.05, 1-tailed test), as well as in 25(OH)D3 and 24,25(OH)2D3, were observed.(ABSTRACT TRUNCATED AT 250 WORDS)