Introduction: The production of nitric oxide is an important peritoneal defense mechanism. We have evaluated the effect of various putative stimulants on nitric oxide production by peritoneal mesothelial cells.Methods: Wistar rats were randomized to either a control group or a peritonitis group (5 mg zymosan intraperitoneally). Groups of five animals were sacrificed at 4, 18, 24, 48 and 96 h after the induction of peritonitis and their peritoneal fluid was harvested for assay. Cultures of peritoneal mesothelial cells were stimulated with lipopolysaccharide, myeloperoxidase, TNFalpha, zymosan, peritoneal fluid from a control animal and peritoneal fluid from a peritonitis animal. Supernatants were collected after incubation for 4, 24 and 48 h for assay. The assay for nitric oxide was based upon the nitrite content of the samples.Results: The intraperitoneal administration of zymosan was associated with an increased production of nitric oxide (NO) when compared with control animals (P < 0.01). In cultures of peritoneal mesothelial cells, zymosan, but not the other putative stimulants, was associated with a marked output of nitric oxide (P < 0.001).Conclusion: Zymosan has a direct effect on peritoneal mesothelial cells, which are able to generate nitric oxide in the absence of co-stimulatory molecules. This suggests that it may be possible to use some form of external stimulation to up-regulate the NO response by peritoneal mesothelial cells.