Pathophysiologic mechanisms involved in the dextran effect on mortality in intestinal shock in rats were studied, using a standardized model for intestinal vascular obstruction. Both dextran 70 and dextran 40 (respective mean molecular weights 70 X 10(3) and 40 X 10(3) d) given to shocked rats reduced but did not prevent the cardioinhibitory action of intestinal venous plasma in vitro. Both dextrans prevented platelet trapping in the lungs. Reduction in blood and plasma volumes was found in shocked rats given saline or dextran 40, but not following dextran 70. These differences were found after 120 min, but not after 240 min, when all shocked groups had lower blood and plasma volumes than in non-shocked controls. The results indicate that several mechanisms influence the effect of dextran 70 on mortality of rats in intestinal shock. Previously observed difference in mortality rates after infusion of dextran 70 and dextran 40 could not be explained by mechanisms studied in this series of experiments.