The effects of cold ischemia on the loop of Henle, distal tubule and collecting duct of the rat kidney were studied by light and electron microscopy in vitro and also in vivo after transplantation. The duration of cold ischemia was chosen so that after transplantation the function of the kidneys was normal (2 h of ischemia), greatly reduced (12 h) and almost abolished (16 h). The results showed that the cold ischemia led to alterations in the thin loop of Henle and distal tubule in vitro which were reversible, judging from the fine-structural appearance. In the thick ascending limb of the loop of Henle and the collecting duct there were alterations in some cells that could have been signs of irreversible cell damage, but probably were fixation artifacts. In vivo 24 h after transplantation of kidneys exposed to 12 and 16 h of cold ischemia extensive necrotic areas comprising loops of Henle and collecting ducts were seen in the inner stripe of the outer zone of the medulla, and to some extent in the inner zone of the medulla. In segments with viable cells the only changes were proteinaceous casts and cell debris in the lumina and a moderate number of intracellular absorption droplets, especially in the medullary part of the collecting duct. It seems probable that the functional impairment in these groups was caused by the widespread necrotic areas in the inner stripe and inner zone, which were presumably the result of an unsatisfactory reflow of blood to these regions after transplantation, a “no reflow” phenomenon.