In order to evaluate peritoneal membrane function and responsiveness of peritoneal microcirculation to vasoactive agents in long-term continuous ambulatory peritoneal dialysis (CAPD) patients, we studied peritoneal clearances of urea (Curea) and creatinine (Ccr), protein concentrations in drained dialysate (DPC), peritoneal glucose absorption (% GA), and drained dialysate volume (VD) before and after nitroprusside (NP) addition to dialysis solution in 17 long-term CAPD patients (mean duration of CAPD: 52 months) and the results were compared to those of 18 patients who were just trained for CAPD (mean duration: 0.6 month). There were no differences in the control (without NP) Curea, Ccr, DPC, %GA, and VD between the new and long-term CAPD patients. Curea, Ccr, and DPC increased significantly with NP in both new and long-term patients. Curea and Ccr with NP were not different between the new and long-term patients but DPC with NP was significantly lower in the long-term CAPD patients. The results of this study suggest that peritoneal solute clearances and the responsiveness of peritoneal microcirculation to NP remain unchanged after four years of CAPD, despite recurrent episodes of peritonitis.