Objective To assess the effect of an inhibitor of nitric oxide synthesis [NG-nitro-L-arginine methyl ester (L-NAME)] on peritoneal transport during peritoneal dialysis (PD) and peritonitis in rats. Methods The authors studied peritoneal transport of small and large solutes, and net ultrafiltration (UF) in rats during PD with Dianeal 3.86 (Baxter, McGaw Park, IL, U.S.A.). They evaluated the effect of L-NAME used as an additive to dialysis fluid in concentrations 0.5 -5 mg/m L on peritoneal transport of small and large molecules and on transperitoneal UF. In addition, they studied the effect of L-NAME (5 mg/mL) during acute peritonitis induced by lipopolysaccharides (5 μg/mL) given intraperitoneally. Results The addition of L-NAME to dialysis fluid increased the selectivity of the peritoneum and net UF during dialysis. Lipopolysaccharides used as an additive to the dialysis fluid, together with L-NAME, did not induce changes in transperitoneal transport of small and large solutes and did not cause a significant decline in net UF. L-NAME given intraperitoneally reduced both local and systemic production of nitric oxide, which might explain its effects on peritoneal transport. Conclusions Nitric oxide is an important mediator of changes in peritoneal transport and its effect is especially significant during peritonitis.