Background: During peritoneal dialysis, mesothelial cells are chronically exposed to high concentrations of glucose. Therefore, the cytotoxic effect of glucose may alter the function and reactivity of these cells. Methods: For 4 weeks, human peritoneal mesothelial cells were cultured in vitroin medium supplemented with 45 m M glucose or 45 m M mannitol or with 45 m M glucose and 1 m M L-2-oxothiazolidine-4-carboxylic acid (OTZ), the latter being a precursor for glutathione synthesis. Peroxidation of the mesothelial cell lipids, synthetic activity and reaction of these cells to peritoneal dialysis fluids were studied. Results: In contrast to mannitol, glucose enhanced the peroxidation of the cellular lipids (+65%, p < 0.01) an effect that was prevented by OTZ. Synthesis of hyaluronan and vascular endothelial growth factor was reduced in mesothelial cells treated with glucose by 36% (p < 0.01) and 44% (p < 0.05), respectively; both glucose effects were reversed when cells were incubated with glucose plus OTZ. Monocyte chemoattractant protein-1 synthesis by cells exposed to glucose was increased by 31% (p < 001), and again that effect was prevented by OTZ. Glucose and mannitol stimulated synthesis of fibronectin (+32%, p < 0.05). Mesothelial cells chronically exposed to glucose became activated after subsequent exposure to the dialysis fluid, as reflected by the increased release of interleukin (IL)-6, in contrast to control mesothelial cells, in which IL-6 synthesis was suppressed. Conclusions: Chronic exposure of mesothelial cells to glucose changes their synthetic activity and their reaction after exposure to dialysis fluids. Some of these effects are prevented by OTZ, which suggests that glucose-induced free radicals are responsible for a change in mesothelial cell phenotype under the conditions of peritoneal dialysis.