The plasma membrane fractions from separated cortex and stele of primary roots of corn (Zea mays L. WF9 x M14) contained cation ATPase activity at similar levels but with somewhat different properties. ATPase activity from cortex was optimum at pH 6.5, showed a simple Michaelis-Menten saturation with increasing ATP.Mg, and showed complex kinetic data for K(+) stimulation similar in character to the kinetic data for K(+)-ATPase and K(+) influx in primary roots. The results for cortex indicate that homogenates of primary roots are dominated by membranes from cortical cells.ATPase activity from stele was optimum at pH 6.5 and showed another maximum at pH 9. At pH 6.5, activity from stele had properties similar to that from cortex except that the kinetics of K(+) stimulation closely approached that expected for a Michaelis-Menten enzyme. At pH 9, the enzyme activity from stele was inhibited by 5 mug/ml oligomycin, suggesting that a significant portion of the activity was of mitochondrial origin. Sucrose density gradient analysis indicated some contamination of mitochondrial membranes in the plasma membrane fraction from stele. The results for stele are consistent with the view that stelar parenchyma cells are not deficient in ion pumps.