The mechanism by which gentamicin augments the uptake of p-aminohippurate (PAH) by rat renal cortical slices was investigated. In all experiments, gentamicin was administered as gentamicin sulfate at 100 mg/kg b.wt. per day for 2 days; control rats were injected with saline. The effect of gentamicin on the metabolism of PAH to p-aminobenzoic acid (PABA), acetyl-PABA and acetyl-PAH was studied by high performance liquid chromatography. No metabolites of PAH were detected in renal slices of gentamicin-injected or control rats incubated in medium containing PAH. Efflux of 14C-PAH was measured after incubating renal cortical slices for 2 hours in medium containing 8 X 10(-5) M 14C-PAH. The efflux rate constant was 0.080 +/- 0.003/min in control slices and 0.059 +/- 0.003/min in gentamicin slices, P less than .001. No significant difference in the diffusible pool of PAH was found between the two groups which supports an argument against increased tissue-binding of PAH as the explanation for the augmented uptake of PAH by slices of gentamicin-injected rats. Active PAH transport was assessed in terms of Michaelis-Menten kinetics. Vmax was 0.93 +/- 0.08 micronmol/g/15 min in control slices and 1.37 +/- 0.10 micronmol/g/15 min in gentamicin slices (P less than .005). The apparent reaction rate constant (Km) was not different; Km was 0.25 +/- 0.03 and 0.29 +/- 0.04 mM in control and gentamicin slices, respectively (P less than .4). In contrast to PAH, gentamicin did not alter uptake of N'-methynicotinamide, an organic base; nor did it alter the efflux rate constant or diffusible pool of N'-methylnicotinamide. Increased PAH uptake was still evident when slices of gentamicin-injected rats were incubated in medium without acetate. These studies indicate that gentamicin stimulates active PAH transport and decreases PAH efflux in rat renal cortical slices. Both changes implicate an effect of gentamicin at the antiluminal membrane of proximal tubular cells. The finding of an increase in Vmax without a change in Km raises the possibility that gentamicin increases the amount or availability of carrier protein-mediating PAH transport.