Despite a wide range of available surgical interventions, the optimal strategy for perforated pyloroduodenal ulcers, including cases combined with other complications, remains undetermined. The use of conventional techniques in patients with peritonitis is often associated with purulent-septic complications. The aim of the work was to investigate the pathophysiological features of peritonitis in perforated pyloroduodenal ulcer and their influence on the course of this disease. The study included 158 patients aged 20-73 years, divided into three groups according to the duration of peritonitis: up to 6 h (n = 99), 6-12 h (n = 35), and more than 12 h (n = 24). All patients underwent bacteriological examination, measurement of abdominal exudate pH at different stages of peritonitis, and analysis of clinical and laboratory parameters. The duration of peritonitis was found to be associated with a higher degree of bacterial contamination of the abdominal cavity. Within 60 minutes after perforation, the initially acidic gastric contents in the abdominal cavity become neutralized; at an exudate pH of 7, optimal conditions for pathogenic microflora are established. Thus, the presented clinical, laboratory and bacteriological results indicate that during the first 3-5 days of the early postoperative period, the patient' s body continues to fight the consequences of post-perforation peritonitis.