The hypersensitive response (HR) is defined as rapid cell collapse at the infection site and often accompanies plant resistance. The physiological processes leading to HR are not well understood. Here, we report an electrophysiological characterization of bacterial HR caused by a single avirulence gene in the absence of other bacterial signals. We used dexamethasone (dex)-inducible transgenic Arabidopsis (Arabidopsis thaliana) plants containing the avrRpt2 gene from Pseudomonas syringae pv tomato. Membrane depolarization in these plants began 1 to 1.5 h after dex application, hours before electrolyte leakage. Progressive depolarization was a sensitive early indicator of HR that occurred only in Arabidopsis leaf cells expressing both avrRpt2 and a functional RPS2 gene. Hyperpolarization of fully depolarized membranes by fusicoccin, a fungal toxin that activates the H(+)-ATPase, indicates that depolarization did not result from a nonfunctional pump or leaky membranes. Depolarization and electrolyte leakage were inhibited in RPS2 plants by the calcium channel blocker LaCl(3), highly correlating these events and suggesting that Ca(2+) entry into cells is required for both. Also correlated were inhibition of depolarization, electrolyte leakage, and HR following salicylic acid pretreatment. In salicylic acid-pretreated RPS2 seedlings, avrRpt2 transcript was produced after dex treatment. However, AvrRpt2 protein accumulation was greatly reduced, suggesting a possible mechanism for inhibition of HR in plants with induced resistance. This experimental system is a very sensitive assay that lends itself to the dissection of physiological processes leading to HR in plants, and provides a baseline for future research within a genetic framework.