Plasma membrane-enriched vesicles were isolated from maize roots, and pH-induced changes of the physical state of the plasma membrane were monitored with a fluorescence probe, TMA-DPH. At physiological temperatures, the membrane's microviscosity increased upon acidification of the vesicles' suspending medium. It seemed that membrane protonation led to a reorganization of maize root plasma membranes. ATP-dependent proton pumping activity across the vesicular membrane was assayed by quinacrine fluorescence quenching. By comparing the steady-state level of pumping activity, the proton pumping activity at pH 6.5 was about three times that determined at pH 5.5. Proton pumping apparently originated in a plasma-membrane bound Mg 2+ -ATPase.