The inhibitory effect of bumetanide on the accumulation of p-aminohippurate (PAH) in rat kidney slices was investigated. Bumetanide increased the Km value for PAH while the Vmax remained to be constant, indicating a competitive inhibition of PAH uptake by bumetanide in the slices. Addition of bumetanide to the medium increased the efflux of PAH with which the slices were preloaded. These data suggest that the inhibitory effect of bumetanide on PAH accumulation is due to an decrease in the PAH transport to the slices from the medium as well as an increase in the efflux of PAH. The present study suggests that the effect of bumetanide was not mediated by the changes of the intracellular contents of electrolytes, Na+, K+, Ca2+ and Mg2+.