Abstract Active accumulation of p- aminohippurate by cortex slices of rabbit kidney in vitro requires 2.5–3 h at 25°C to attain a steady-state value. The uptake process can be divided into a rapid and a slow component. The slow component makes the largest contribution to the final accumulation of p- aminohippurate . Rapid uptake is dependent on oxidative metabolism, but is not affected by Na + or ouabain. By way of contrast, Na + is required for the slow uptake process to occur. Efflux of p- aminohippurate accumulated in the presence of Na + is stimulated by transfer to media with a low concentration of Na +. On the other hand, there is only evidence of a stimulatory effect of Na + on p- aminohippurate influx at medium concentrations above 30 mM. The results indicate the existence of both Na +-dependent and Na +-independent p- aminohippurate transport, respectively.