The effects of phytohormones (abscisic acid, gibberelic acid, and jasmonic acid) and ambiol (a synthetic growth regulator) on processes of H+ transport across the plasmalemma were studied in membrane vesicles isolated from the parenchyma of potato (Solanum tuberosum L.) tubers. Phytohormones and ambiol were tested either individually or in combinations. Each of the substances tested changed the initial rate of H+ uptake by the vesicles. Two signaling substances added to the incubation medium simultaneously modified the activity of each other. It is suggested that the interaction of a signaling substance with components of the plasmalemma alters the responses of the membrane to other signaling molecules.