The effect of protein binding on kidney function has been studied by investigating the renal accumulation and secretion of the hippurate analogue 2-methylbenzoylglycine in the isolated perfused rat kidney in the absence and presence of bovine serum albumin (BSA). Experiments were performed with either 2.5% pluronic or a combination of 2.2% pluronic and 2% BSA as oncotic agents; a wide concentration range (1-190 micrograms mL-1) of 2-methylbenzoylglycine was studied. Tubular secretion appeared to be a function of the amount of unbound drug in the perfusate and was best described by a model consisting of a high and low affinity Michaelis-Menten term. Parameters obtained after the analysis of renal excretion data were maximum transport velocity for the high affinity site (TM,H) = 3.0 +/- 2.8 micrograms min-1, Michaelis-Menten constant for tubular transport for the high affinity site (KT.H) = 0.5 +/- 0.8 microgram mL-1, maximum transport velocity for the low affinity site (TM,L) = 250 +/- 36 micrograms min-1, and Michaelis-Menten constant for tubular transport for the low affinity site (KT,L) = 62 +/- 17 micrograms mL-1. The compound accumulated extensively in kidney tissue, ratios up to 175 times the perfusate concentration were reached. Accumulation data were best analysed by a two-site model similar to the model used to describe renal excretion. Calculated parameters were theoretical maximum capacity of the high affinity site (RM,H) = 26 +/- 23 micrograms g-1, affinity constant for renal accumulation at the high affinity site (KA,H) = 0.2 +/- 0.4 microgram mL-1, theoretical maximum capacity of the low affinity site (RM,L) = 1640 +/- 1100 micrograms g-1 and affinity constant for renal accumulation at the low affinity site (KA,L) = 60 +/- 58 micrograms mL-1. The very high accumulation in kidney tissue could be explained by active tubular uptake, mediated by the secretory mechanisms involved, and dependent on the amount of free drug in the perfusate. This study shows that anionic drugs, subject to active secretion, may reach high concentrations in tubular cells even at low plasma concentrations.