The effects of medium K+ concentration on the tissue uptake of p-aminohippurate (PAH) were investigated in rat kidney cortex slices. K+ concentration in the incubation medium was changed in the absence of Na+. Increase in K+ concentration up to 145 mM revealed an augmenting effect on PAH uptake when tissue water content was controlled at about 72% by elevating medium osmolarity with mannitol. The K+-induced enhancement of PAH uptake was almost completely abolished by the simultaneous presence of extremely high concentration of hippurate. Anoxia reduced this augmenting effect. Pyruvate added to the medium scarcely affected this augmenting effect. The uptake rate during the initial 10-min incubation increased linearly in proportion to the increase in log [K]o/[K]i. The results suggest that high medium K+ concentration stimulates PAH transport by a carrier-mediated process which is dependent on oxidative metabolism. However, the mode of action of K+ on PAH uptake seems to be different from that of Na+. It seems likely that K+ exerts its effect on PAH transport by the alteration of the peritubular membrane potential.