Diffuse peritonitis is a common complication after open abdominal surgeries, prostatic or intestinal disorders. Different pharmaceutical or surgical methods have been used for treatment of peritonitis, but abdominal drainage has a special role in the treatment of peritonitis. In this study, 15 dogs of different breeds that suffered from diffuse peritonitis following abdominal surgery were allocated into three groups. Group A was treated with systemic Ampicillin (20mg/kg, q6h) and Gentamycin (2.2mg/kg q8h). Group PD, in addition to systemic antibiotics, were treated by insertion of abdominal Penrose drains and abdominal washing. Group SPD, in addition to systemic antibiotics, were treated by insertion of an abdominal Sump-Penrose drain and abdominal washing. All animals were closely observed for 12 days after treatment. Heart rate, temperature, white blood cell counts and differentials, and mortality were monitored. Use of the Sump-Penrose drains plus abdominal washing was the most satisfactory method for treatment of diffuse peritonitis.