Rats were infused intraaterially with 10(9) live E coli, or 10(10) live B fragilis, or 0.9% NaCl solution. Within 6 hr or sooner E coli-treated rats extensive hepatic hypoxia (mean hepatic pO2 less than 5 mm Hg), hypoglycemia, lactacidemia, and microscopic evidence of sinusoidal damage and hepatocyte hypoxia. Mortality was 100% within 24 hr. Rats which received B fragilis showed significantly less hepatic hypoxia (mean hepatic pO2 13.8 mm Hg) and lactacidemia and were euglycemic at 6 hr. Hepatic tissue showed congested sinusoids and depletion of glycogen. Mortality was 0% in 24 hr. NaCl-treated rats were stable over 6 hr of observation; mean hepatic pO2 was 20.6 mm Hg. We suggest that the events associated with acute lethal E coli bacteremia are related in part to strutural interference of the hepatic microcirculation. Although the effects of B fragilis bacteremia were appreciably less pronounced, the ability to establish hypoxic foci may have important implications.