The peritoneal environment during infection. The effect of monomicrobial and polymicrobial bacteria on pO2 and pH. Intraperitoneal (IP) abscesses frequently are composed of aerobes and anaerobes, and, in experimental models, a particulate adjuvant. The environmental changes effected by these components, either singularly or in combination, have not been well defined. The IP pO2, pH, and recoverable bacteria from the peritoneum of rats were quantified over 6 hours during simple aerobic and anaerobic infections and during mixed peritonitis with and without a sterile feces-barium sulfate adjuvant (SFA). Fourteen groups were studied, receiving intraperitoneally, at time of oxygen probe placement, 1 mL normal saline (control), Escherichia coli (EC), Bacteroides fragilis (BF), SFA alone, or a mixture of EC and BF, EC and SFA, BF and SFA, or EC, BF, and SFA. Control animals exhibited a stable IP pO2 and pH during 6 hours. In monomicrobial EC peritonitis, inocula well below the LD50 produced an increased IP pO2 and reduced arterial-peritoneal gradient (APG), with a stable IP pH. By 6 hours lethal doses of EC produced a dramatic decline in IP pO2, with no change in arterial pO2 as well as acidic IP and arterial pHs. Simple BF peritonitis caused no or minor elevations in IP and arterial pO2 with no change in pH. During mixed infections a significant decline in the IP pO2 and pH at 6 hours in those groups infected with both SFA and EC of a moderate, normally sublethal inoculation was observed, while arterial pO2 was unchanged and arterial pH was decreased only slightly. Concomitantly there was a significant increased number of aerobic bacteria in those groups with SFA as adjuvant compared to similar inocula without SFA. This study demonstrates the complex interactions of bacteria, sterile particulate adjuvant (SFA), and the host peritoneum. It suggests that the combination of SFA and aerobic bacteria alter the peritoneal environment to one permitting anaerobic growth and promoting abscess formation. 