1. Some parameters of calcium and phosphorus metabolism and the radioimmunoassay of plasma concentrations of both the carboxyl (COOH) (residues 53-84) and amino (NH2) terminal (residues 1-34) fragments of parathyroid hormone (PTH) were measured to evaluate secondary hyperparathyroidism in 68 patients with chronic renal failure (CRF), 34 of whom were on hemodialysis therapy. 2. The upper limits of the normal values for serum PTH-NH2 and PTH-COOH concentrations were 28 and 146 pmol/l, respectively. Patients with mild CRF (plasma creatinine (CRp) 1.2-2 mg/dl) had normal mean serum total calcium, low mean serum phosphorus, undetectable plasma levels of PTH-NH2 (less than 10 pmol/l), slightly elevated mean plasma PTH-COOH concentration and normal fractional excretion of phosphorus (FEP). Patients with moderate CRF (CRp 2.1-4 mg/dl) had normal mean serum concentrations of both total calcium and phosphorus, and elevated mean levels of both plasma PTH-COOH and PTH-NH2 associated with increased FEP. Patients with end-stage CRF (CRp greater than 4 mg/dl) and those on hemodialysis had elevated mean serum phosphorus levels and decreased mean serum total calcium concentrations compared with those with mild and moderate CRF, and more pronounced increases in both mean plasma PTH-COOH and PTH-NH2. 3. The logarithm of plasma PTH-NH2, but not PTH-COOH, concentration correlated positively with FEP and serum phosphorus concentration and negatively with total serum calcium concentration, while the logarithms of both PTH-NH2 and PTH-COOH levels correlated positively with CRp.(ABSTRACT TRUNCATED AT 250 WORDS)