Ischemia was produced in the inner cortex of the rat kidney by clamping the pedicle (artery, vein and ureter) and severing collateral connections. After 30 min of ischemia, a slight aggregation of membrane-associated particles was observed in the freeze-fractured plasma membranes. The aggregation was progressive after 60 and 120 min of ischemia. These changes were reversible after 15 and 240 min of reflow of blood following 30 or 60 min of ischemia. The changes were irreversible after 120 min of ischemia. The cells were vacuolated after ischemic periods of 30 min or longer and after 120 min of ischemia the tissue was severely damaged and aggregation of membrane-associated particles was evident in the vacuolar membranes. No changes in the tissue or plasma membrane were observed after 5 or 20 min of ischemia.