Laparoscopic surgical repair of perforated gastroduodenal ulcer is technically feasible. To study the effect of a pneumoperitoneum on the extent and severity of peritonitis this animal study was devised. In rats gastric ulceration was induced by instillation of ethanol (50%, 2 ml) and followed by gastrotomy to simulate perforation. Animals were randomly allocated to pneumoperitoneum (PP) and control groups. In PP groups CO2 was insufflated intraperitoneally 6, 9, 12, and 24 h after gastrotomy. In controls the abdomen was only punctured. Animals were sacrificed 5 h after the end of PP or abdominal puncture. Blood cultures and intraabdominal swabs were assessed. A peritonitis severity score (PSS) based on histologies from peritoneum, liver, left kidney, spleen, and first jejunal loop was estimated. Six and 9 h after gastrotomy no significant differences between the PP and control groups were observed; 12 h after gastrotomy cultures of blood samples and abdominal swabs were positive in 67% and 75% in the PP group compared to 42% (P <0.05), and 42% (P <0.05) in controls. The mean PSS was 20.8 (standard deviation [SD] 2.2) in the PP group compared to 11.3 (1.5) (P <0.01) in controls; 24 h after gastrotomy cultures of blood samples and abdominal swabs were positive in 83% and 100% in the PP group compared to 42% (P <0.05) and 50% (P <0.01) in controls. The mean PSS was 22.1 (1.5) in the PP group compared to 11.8 (2.4) (P <0.01) in the controls.(ABSTRACT TRUNCATED AT 250 WORDS)