Methods Observations are reported on a total of 37, male, Sprague-Dawley rats studied at intervals of either 1, 2, 4, or 8 weeks after unilateral acute renal failure was produced by clamping the left renal artery for 1 hour. Prior to clamping, unfasted animals had been anesthetized with sodium pentobarbital, 60 mg/kg of body weight, placed on a heated table that maintained body temperature between 37 C and 38 C, and their left kidneys exposed through a small subcostal incision. The left renal artery was isolated, 10 to 15 U/l00 g body weight of heparin (1,000 U/ml, The Upjohn Co., Inc.) was injected intravenously, and a small, smooth surfaced tension clamp (Schwartz 1-inch clip, Roboz Surgical Instruments, Co., Inc.) was positioned on the artery so that it was completely occluded. The clamp was removed after 1 hour, the incision closed, and the rats re-turned to their cages. Saline (0.85% sodium chlo-ride) was infused intravenously at 40 p. 1/mm for 1 hour before and after occlusion; the rate of infusion during the ischemic period was reduced to 20 pI/mm.