Plasma membrane vesicles were purified from 8‐day‐old oat (Avena sativa L. cv. Brighton) roots in an aqueous polymer two‐phase system. The plasma membranes possessed high specific ATPase activity [ca 4 μmol P₁ (mg protein)⁻¹ min⁻¹ at 37°C]. Addition of lysophosphatidylcholine (lyso‐PC) produced a 2–3 fold activation of the plasma membrane ATPase, an effect due both to exposure of latent ATP binding sites and to a true activation of the enzyme. Lipid activation increased the affinity for ATP and caused a shift of the pH optimum of the H⁺ ‐ATPase activity to 6.75 as compared to pH 6.45 for the negative H⁺‐ATPase. Activation was dependent on the chain length of the acyl group of the lyso‐PC, with maximal activition obtained by palmitoyl lyso‐PC. Free fatty acids also activated the membrane‐bound H⁺‐ATPase. This activation was also dependent on chain length and to the degree of unsaturation, with linolenic and arachidonic acid as the most efficient fatty acids. Exogenously added PC was hydrolyzed to lyso‐PC and free fatty acids by an enzyme in the plasma membrane preparation, presumably of the phospholipase A type. Both lyso‐PC and free fatty acids are products of phospholipase A₂ (EC 3.1.1.4) action, and addition of phospholipase A₂ from animal sources increased the H⁺‐ATPase activity within seconds. Interaction with lipids and fatty acids could thus be part of the regulatory system for H⁺‐ATPase activity in vivo, and the endogenous phospholipase may be involved in the regulation of the H⁺‐ATPase activity in the plasma membranne.