The authors compared three groups of subjects: - the first group (11 patients) : re-operated for post-operative peritonitis; - the second group (6 patients) : re-operated for evisceration without any underlying lesion; - the third group (8 patients) : non-reoperated, complicated abdominal surgery. Study of the different laboratory parameters helped to identify a number of changes which could plead in favor of reintervention, in the group of peritonitis. Among these modifications, the progressive fall in the urinary Na/K ratio and the negativity of the nitrogen balance appear to play a privileged role.