Potassium stimulation of the plasmalemma (Zea mays L. var Mona) was studied by using a constant ionic strength to prevent indirect stimulation by the electrostatic effect of K(+) salts. The transmembrane electrochemical H(+) gradient was eliminated by using gramicidin. In these conditions, K(+) stimulation was attributable to a direct effect of the cation on plasmalemma proteins. We used both native vesicles isolated on a sucrose cushion, and solubilized and purified ATPase from phase-partitioned plasmalemma, according to the method of T. Nagao, W. Sasakawa, and T. Sugiyama ([1987] Plant Cell Physiol 28: 1181-1186). The purified enzyme had a high specific activity (15 micromoles per minute per milligram protein), but was only about 20% stimulated by K(+). In both preparations, potassium (in the range around 1 millimolar) specifically decreased two-fold the vanadate inhibition constant, and increased the maximum rate of ATP hydrolysis. In plasmalemma vesicles, the Eadie-Scatchard graph of the K(+)-dependent ATPase activity as a function of K(+) concentration was linear only at constant ionic strength. The purified ATPase preparation appeared as two closely spaced bands in the 100 kilodalton region with isoelectric point about 6.5. Nevertheless, this biochemical heterogeneity seems unlikely to be related to K(+) stimulation, since K(+) modified neither the pH optimum of the activity (pH 6.5) nor the monophasic kinetics of the vanadate inhibition, in both native plasmalemma and purified enzyme preparation.