The evolution of serum iPTH concentration during hemodialysis was studied in eight patients who were dialyzed with cuprophane (Cu) and polyacrylonitrile membranes (PAN) during two four-hour sessions. Ca+(+) concentration in the dialysate was 1.37 mM/liter. iPTH was measured with an intact hormone immunoradiometric assay (I), with two late (L1, L2) and one mid (M) carboxylterminal immunoassays at the beginning and end of hemodialysis, from the arterial and venous sides of the extracorporeal unit. Results are means +/- SD. Serum Ca+(+) increased during dialysis with Cu (1.26 +/- 0.08 vs. 1.33 +/- 0.03 mmol/liter, P less than 0.05, without any change in the concentration of iPTH measured with L1, L2 or M, but with a 50% decrease in iPTH measured with I (21.8 +/- 19.2 vs. 10.3 +/- 9.0 pmol/liter, P less than 0.05). Serum Ca+(+) increased similarly with PAN (1.25 +/- 0.10 vs. 1.34 +/- 0.04 mmol/liter, P less than 0.01), but there was a greater than 50% decrease in iPTH concentration measurements for all four assays (I: 17.2 +/- 17 vs. 7.6 +/- 8.3 pmol/liter, P less than 0.05; L1: 92 +/- 75 vs. 36 +/- 32 pmol/liter, P less than 0.05; L2: 312 +/- 289 vs. 126 +/- 128 pmol/liter, P less than 0.01; M: 926 +/- 1074 vs. 373 +/- 422 pmol/liter, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)