Addition of many oxidizable substrates to medium often enhances p-aminohippurate (PAH) transport by incubating renal tissue. Since the oxidation of various substrates by rat kidney tissue may change medium pH and this pH change has not been considered by many in conclusions, we followed the effects of medium pH on the magnitude and kinetics of in vitro PAH transport. Two previous studies employing rat kidney slices reported different optimal medium pH for PAH accumulation, i.e. pH 6.5 and pH 8.0. By repeating these studies over a wide range of medium pH, we confirmed the presence of enhanced steady state accumulation relative to pH 7.4 at two separate pH ranges, one at pH 6.7–6.9 (+15%), and the other at pH 7.7–7.9 ( + 22%). PAH influx, as measured by early tissue accumulation, was not different at pH 7.4 and 7.8; while at pH 6.8, it actually decreased. It was only later in the incubation that accumulation was greater at pH 6.8 and 7.8. Probably due to the small stimulation in steady state accumulation at either pH, we could not discern, by the methodology currently available, if this was secondary to augmented influx, decreased efflux, or a combination of both in the latter part of incubation. We conclude that the contribution secondary to medium pH changes must be considered when evaluating the effects of various substrates on PAH accumulation by kidney slices.