Intracellular electrical recordings in onion (Allium cepa L.) guard cells show that they maintain a membrane potential difference (MPD), inside negative. The MPD of intact cells averaged -72_+ 29 mV (n = 45); MPD of cells partially digested with a cellulolytic enzyme, -39 +_ 7 mV (n = 65). Evidence indicates that the guard cells have two electrically distinct compartments, presumably delimited by the plasmalemma and tonoplast. Epidermal cells in partially digested preparations also showed MPD that could be either positive ( + 15 + 7 mV ; n = 23) or negative ( -15 + 8 mV; n= 13). Guard cells exposed to light-dark cycles hyperpolarized in the light and depolarized in the dark. The largest observed voltage changes reached 52 mV during hyperpolarizations and 60 mV during depolarizations. The light responses saturated with roughly exponential kinetics, with the depolarizations exhibiting a slower second phase that might be related to the contracting movements of the guard cells. Initial rates of the responses averaged about 14 mVmin-1 in the dark and about 8 mV min-1 in the light. The results can be interpreted as electrical correlates of fluctuations in intracellular potassium concentration, as light-induced changes in membrane permeability, or as the photoactivation of an electrogenic proton pump. The last possibility seems to be the simplest interpretation of the data that also provides us with a mechanism driving the ion fluxes associated with stomatal function.