Two uptake-wash regimes were used to determine influxes of 86Rb across the plasmalemma and across an internal membrane of maize roots. By using a variety of different conditions it was possible to determine the magnitude of non-metabolic uptake at different concentrations and hence measure more accurately the magnitude of the active influxes at each membrane. This data was used to study possible correlations between ion influxes and the activities of two different membrane-bound KCl-stimulated ATPases isolated from maize roots. One of these ATPase activities was associated with a fraction enriched in plasmalemma and the other with a fraction containing a smooth internal membrane. Significant correlations were found between the influx of 86Rb across the plasmalemma and the plasmalemma-associated, KCl-stimulated ATPase activity and between the influxes and KCl-stimulated ATPase activities associated with an internal membrane. These correlations may be regarded as evidence for mediation of specific ion influxes by ion-stimulated ATPases. However, a number of significant cross-correlations could also be made (e.g. between the influx across the plasmalemma and the inner membrane ATPase activity) which, together with problems of identification of the internal membrane and accurate flux measurement, make it difficult to interpret the result unequivocally in terms of the above hypothesis.