Canine kidneys were subjected to continuous nonpulsatile perfusion using 200 ml of a perfusate containing 50 g/l albumin. When optimal oxygenation was achieved, perfusate K+ contents were unchanged for 24 h, indicating adequate membrane function but tended to increase thereafter. Lowered oxygen pressures resulted in significant cellular K+ loss during the first hours of perfusion. During oxygenated perfusion, glucose and free fatty acids (FFA) were oxidized in considerable amounts with a preferential consumption of octanoate. A capacity for long-chain FFA oxidation became obvious when the octanoate had been used up, but the amount of these FFA in the perfusate depended preferentially on FFA being liberated from tissue lipids during the 1st day of perfusion. Glucose consumption rates were highest during the first 2 days of perfusion but the subsequent reduction of the metabolic rate was not accompanied by an accumulation of lactate. Thus medium-chain FFA and glucose should be supplied to the continuously perfused kidney in hypothermia and optimal oxygenation of the perfusate should be guaranteed. However, it seems to be unnecessary to supply exogenous long-chain FFA.