The metabolism and the washout of 4-hydroxynonenal (HNE), a major lipid peroxidation product, was determined in jejunal intestine of rats under normoxic, ischemic and postischemic conditions. The HNE utilization was by several orders higher than the HNE washout. Under ischemic conditions a drastic decrease of HNE utilization was found, compared with normoxic and postischemic utilization rates. The real flux rate of intracellular HNE formation during ischemia and reperfusion of small intestine was for the first time estimated on the basis of tissue HNE concentration, HNE utilization rate at defined HNE tissue concentration and HNE washout by perfusate circulation. Such estimation led to intestinal HNE formation rates of about 20 nmoles/g/min at normoxia, 20 to 40 nmoles/g/min at ischemia, 100 nmoles/g/min at 10 min of reperfusion and 20 nmoles/g/min at 60 min of reperfusion. Therefore, one can conclude that the tissue concentration of HNE only partly represents the real HNE formation rate and that the formation rate is of particular pathophysiological importance.