We examined the effect of L-2-oxothiazolidine-4-carboxylate (OTZ) on peritoneal morphology and function of mesothelial cells in rats during peritoneal dialysis (PD). We implanted intraperitoneal catheters into 14 rats and injected the rats twice daily for 4 weeks with standard PD fluid (group C, control; n = 7) or PD fluid supplemented with OTZ 1 mmol/L (group OTZ; n = 7). At the beginning and the end of the study, we collected samples of 4-hour dwell effluents from every rat under sterile conditions. We used those samples to test, in ex vivo conditions, the effect on primary cultures of rat peritoneal mesothelial cells with regard to synthesis of total protein and collagen in mesothelium. After 4 weeks, the rats were humanely killed, and glutathione peroxidase activity in erythrocytes was measured. Morphologic analysis of the peritoneal membrane was also performed. As compared with group OTZ, group C showed intraperitoneal adhesions that were more severe, a higher density of mesothelial cells, a higher density of peritoneal vessels, similar peritoneal thickness, lower activity of glutathione peroxidase, and greater ability of their dialysate effluent to induce synthesis of total proteins and collagen in cultured mesothelial cells. Relationships between various parameters were analyzed. We conclude that OTZ preserves peritoneal morphology and the function of mesothelial cells during PD.