The reabsorption capacity of albumin was determined by a clearance technique following a single load of bovine or rat serum albumin in conscious rats. The maximum reabsorption capacity of both albumin amounted to 5.6--6.2 micrograms/min . 100 g BW and there seemed to be a nonselective mechanism for the two albumins. The tubular reabsorption capacity was reached at normal physiological plasma albumin concentration. The renal excretion threshold of both albumins amounted to 31.8 mg/ml, the sieving coefficient phi was calculated to be 2.0 . 10(-4). The glomerular filtration rate, controlled by the total plasma slope clearance of 51Cr-EDTA, remained unchanged following a load of rat serum albumin (54.8 +/- 12.2 ml/min . m2 body surface) but decreased significantly after the administration of bovine serum albumin.