Activation of a host plasma membrane K(+) efflux/net H(+) uptake exchange by pathogenic pseudomonads plays an important role in the development of hypersensitivity in tobacco (Nicotiana tabacum). Involvement of the plasmalemma H(+)-pumping ATPase in this response was investigated. The exchange response of suspension-cultured tobacco cells to Pseudomonas syringae pv syringae was reduced 90% or more by ATPase inhibitors including vanadate, N-ethylmaleimide, and N,N'-dicyclohexylcarbodiimide. The exchange was also strongly inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone and by slightly alkaline external pH. Respiratory inhibitors such as oligomycin and sodium azide reduced the exchange by 50% to 75%, while glycolysis inhibitors such as sodium arsenite and sodium iodoacetate decreased exchange by approximately 90%. These results suggest that plasmalemma H(+)-ATPase activity is required for the exchange response and that this may reflect a requirement for a plasmalemma pH and/or electrical potential gradient.