After reperfusion of kidneys subjected to a period of warm ischemia, the medulla displays a vascular congestion of erythrocytes, especially in the inner stripe of the outer zone, a phenomenon referred to as "trapping." This trapping causes reflow alterations, thus contributing to postperfusion medullary ischemia. The purpose of the present investigation was to study whether trapping also occurs after reperfusion of kidneys following varying periods of eoM ischemia and to determine if there is any correlation between the degree of cold ischemic injury and the extent of erythrocyte trapping. Rat kidneys stored at + 4 ~ for 0-30 h were transplanted into recipient animals pretreated with a 51Cr-labelled erythrocyte suspension. Twenty minutes after reperfusion, the grafts were removed and microdissected into cortex, outer and inner stripes of the outer medullary zone, and inner zone, respectively. The radioactivity of these specimens was measured, and the erythrocyte content for each specimen was calculated. The results show a maximal trapping for cold ischemia time (CIT) of about 12-15 h. A linear correlation between the amount of trapping and CIT could be found in all parts of the kidney (except for the cortex) for CIT 0-15 h. The best correlation was found in the part where the trapping was most prominent, i.e., in the inner stripe. After CIT of 15 h or more, no correlation could be found. It is suggested, as described in models of warm ischemia, that the obstructions of the capillaries by trapped erythrocytes following reperfusion is of pathophysiological significance for the development of posttransplant acute renal failure. Furthermore, the strong correlation between CIT and the extent of erythrocyte trapping, particularly in the inner stripe, indicates that measurement of erythrocyte trapping Offprint requests to: J. Wahlberg after reperfusion could be a sensitive indicator of the degree of cold ischemic damage.