The effect of different albumin concentrations on whole kidney function has been studied in the isolated perfused rat kidney. When a dialyzer was included in the perfusion circuit, GFR and sodium transport remained nearly constant for 2 h. Perfusion flow rate was directly correlated with albumin concentration. GFR was low (0.56 ml.min-1.g-1) at high albumin concentration (7.6 g/100 ml) and highest (1.38 ml.min-1.g-1) when albumin was omitted. The latter observation differed from earlier findings by other investigators who found a reduced GFR in the absence of albumin. This reduced GFR could be explained by ureteral backpressure, which results from the use of narrow ureter catheters. Fractional sodium reabsorption was approximately 50% irrespective of load when albumin was absent, and reached 90% at a GFR of 0.95 ml.min-1.g-1 when 5-6 g/100 ml albumin was used. At reduced load, fractional sodium reabsorption approached in vivo values of 98%. Potassium secretion was observed at high urinary flow rates when albumin concentration was low; low urinary flow rates at high protein concentration were associated with net potassium reabsorption.