Eight horses with Grade 3b rectal tears of the peritoneal part of the rectum or small colon were treated by a combination of medical therapy and dietary manipulation. All of the horses developed septic peritonitis during the course of treatment. Medical therapy consisted of a combination of penicillin, gentamicin and flunixin meglumine administered parenterally, metronidazole administered orally and liquid paraffin administered by nasogastric tube. Some horses also received fluid and electrolyte therapy i.v., plasma and heparin i.v. All horses were maintained on a laxative diet. Six of the 8 horses recovered. Duration of therapy in the horses that survived was between 2 and 7 weeks. Three of the 6 horses that recovered developed a rectal diverticulum, which did not appear to cause any subsequent medical problems. In the 2 horses that died, the Grade 3 rectal tear progressed to a Grade 4 tear with subsequent faecal contamination of the abdomen. Manual evacuation of the cavity of the rectal tears during treatment and size of the tears were identified as possible causes for the progression of the disease in these 2 horses.