PURPOSE: To investigate morbidity, mortality and microbiological response to fecal peritonitis induced in Wistar rats with permanent bilateral carotid ligation (PBCL). METHODS: Fecal peritonitis was induced in 30 rats, with 10 animals in each group: Group1 - normal young animals; Group2 - normal mature animals; and Group3 - rats with PBCL after four months postoperative follow-up. Peritonitis was induced with 10% stool suspension. Morbidity and mortality were evaluated. The survival animals after seven days were euthanized for tests. For microbiological studies blood were collected from the carotids and right ventricle; and fragments of lung and peritoneum. RESULTS: The morbidity and mortality of young animals were significantly lower than in mature animals with and without PBCL. There was no difference in morbidity and mortality among mature rats with and without PBCL. The diversity of microorganisms producing septicemia was similar to native micro biota of the large bowel. CONCLUSIONS: The immune response was more efficient in young animals, represented by significant less morbidity and no natural mortality. PBLC did not affect morbidity and mortality in mature rats. The immune response to fecal peritonitis has age as an independent predictor.