OBJECTIVE To assess the relationship of WBC counts and partial oxygen tension in the portal vein and caudal vena cava with portal bacteremia, bacteria in the liver, and postoperative morbidity and mortality in dogs with portosystemic shunts. METHODS Prospective study. METHODS 12 clinically normal dogs and 15 dogs with single congenital portosystemic shunts. METHODS Blood was collected from the portal vein and caudal vena cava for aerobic and anaerobic bacterial culture, WBC count, and measurement of partial oxygen tension. Samples of liver tissue, indwelling catheters, and fluids administered IV were also obtained for bacterial culture. RESULTS Bacteria were cultured from samples obtained from the portal vein and caudal vena cava of 1 dog with a shunt and from the caudal vena cava of 1 clinically normal dog; neither dog had postoperative complications. In dogs with shunts and in clinically normal dogs, partial oxygen tension in the portal vein was significantly greater than that in the caudal vena cava. Postoperative complications were identified in 33% of dogs with shunts. Partial oxygen tensions of dogs with shunts with postoperative complications did not significantly differ from those of all dogs with shunts or dogs with shunts without complications. Significant differences in WBC counts were not found when comparing dogs with shunts with and without complications. Anaerobic bacteria were not cultured from the liver of any dog. CONCLUSIONS Leukocytosis, portal bacteremia, and portal hypoxemia were not notable findings in dogs with shunts and were not correlated with postoperative morbidity or mortality.