In this report five cases of septic peritonitis in the dog are discussed. Clinical signs of peritonitis are mostly nonspecific. Both diagnosis and treatment represent a strong challenge to the practitioner. The five dogs were suspected of septic peritonitis based on ultrasound and cytology of the abdominal fluid. Lactate and glucose concentrations were also measured in venous blood as well as in abdominal fluid. The difference in concentration between blood and abdominal fluid for glucose (blood to fluid glucose (BFG) difference) and for lactate (blood to fluid lactate (BFL) difference) were calculated by substracting the abdominal fluid concentrations from the blood concentrations. In four dogs the DFG difference was more than 20 mg/dl, which is diagnostic for septic peritonitis. In all dogs the BFL difference was less than 1.5 mmol/l. All dogs were scheduled for exploratory abdominal surgery. One dog was euthanized during surgery. The abdomen of the other four dogs was primarily closed after the removal of the source of contamination and lavage of the abdomen. All four dogs recovered well.