Recent studies have shown that the mass transfer coefficients (MTC) for low- and middle-molecular-weight substances are maintained at medium terms in patients under continuous ambulatory peritoneal dialysis (CAPD) therapy. We have studied the MTC for parathormone (MW 5,500–6,000) in 29 patients under CAPD. 11 patients were assessed once, 9 patients were assessed twice, and 9 were assessed three times, with 6-month intervals between assessments; in 15 cases nitroprusside (4.5 mg/l) was added to the dialysate, and in 2 cases the assessment was carried out during an episode of peritonitis. A bicompartmental model with exponential ultrafiltration was used, and the generation rates were calculated by mass balance. The MTC was calculated by minimization of the quadratic error, following the procedure of Runge and Kutta. The mean MTC values were 21.2 ± 5.9 for urea, 10.26 ± 5.9 for creatinine 2.8 ± 1.8 for inulin and 1.0 ± 0.8 for parathormone (PTH), with significant correlation between the MTC values for PTH and inulin (r = 0.5, p < 0.01). There is a direct correlation between MTCpth and peritoneal PTH clearance, and an inverse one between MTCpth and serum PTH, with r = 0.8 and 0.51, respectively. No correlation was found between the MTC for inulin or PTH and the duration of the CAPD therapy, nor with the number of episodes of peritonitis. No significant medium-term variations were detected in the MTC values for PTH and inulin. Both peritonitis and the intraperitoneal administration of nitroprusside induce significant increases in the M