BACKGROUND: Ultrafiltration failure resulting from increased peritoneal vascular permeability during peritonitis is a major problem in CAPD patients. However, the mechanism of increased peritoneal permeability during peritonitis has not been clearly determined. We studied changes in the peritoneal permeability and the expression of vascular endothelial growth factor(VEGF), which is known to increase vascular permeability, in the peritoneal tissues of rats with peritonitis. METHODS: After implanting peritoneal dialysis catheter to 20 Sprague-Dawley rats and performing peritoneal equilibration test(PET), rats were divided into control group(n=7) and peritonitis group(n=13). One ml of saline or Staphylococcus aureus(1X10(9) colony forming unit/mL) was injected intraperitoneally to control or peritonitis group for five days, and PET was repeated. Peritoneal transport rates of glucose and total protein were measured, and macrophages infiltration and VEGF expression in peritoneal tissues were examined by immunohistochemical stain. RESULTS: Peritoneal transport rates of glucose and total protein were significantly increased in peritonitis group compared with control group, suggesting that peritonitis increased peritoneal transport rates of both low and high molecular weight solutes. The peritoneal tissues of peritonitis rats showed profuse infiltration of macrophages in the submesothelial space, submesothelial widening and denudation of mesothelial cells. While VEGF was slightly expressed in peritoneal mesothelial cells in control rats, it was intensively stained not only in the peritoneal mesothelial cells but also in the infiltrated macrophages in the submesothelial space in peritonitis rats. CONCLUSION: Peritonitis increases peritoneal vascular permeability and VEGF expression in peritoneal mesothelial cells and infiltrated macrophages in the submesothelial space. These data suggest that VEGF may play a role in the increased peritoneal permeability during peritonitis.