Neutrophils may contribute to the pathogenesis of ischemia-reperfusion injury by impairing microcirculatory flow and causing tissue damage. In this study neutrophil deformability and activation were studied after acute ischemia and reperfusion of the rat hindlimb. A cell transit analyzer with 8 microm pore filters was used to measure the flow resistance of individual neutrophils isolated from arterial blood drawn 30 seconds and 5 minutes after reperfusion, after 2 hours of femoral or iliac artery occlusion, in anesthetized rats. Femoral occlusion followed by 30 seconds of reperfusion resulted in a 16% rise in mean pore transit time, and iliac occlusion caused a 31% increase. Neutrophil activation (percentage of cells with nonspherical shape) rose from 6% to 23% (femoral occlusion) or from 7% to 29% (iliac occlusion). By 5 minutes of reperfusion, transit times, but not shape change, had almost returned to preischemic levels. Sham-occluded rats had no significant change in neutrophil behavior. Plasma isolated from rats after 30 seconds of reperfusion after 2 hours of iliac occlusion caused a 27% increase in mean transit time when incubated with nonautologous neutrophils, while activation rose from 32% to 55%. Thus plasma factor(s) released after ischemia and reperfusion of rat hindlimb caused transient rheologic alteration of circulating neutrophils, which may contribute to local or disseminated vascular occlusion and tissue injury.