Effect of various inhibitors on the P-type Na+-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum, was examined. The ATPase was extremely sensitive to p-chloromercu-riplienylsulfonic acid, a modifier of SH-group. The enzyme was also sensitive to diethylpyrocarbonate, and analysis of the inhibition kinetics by the drug indicated that modification of a single histidine residue per ATPase molecule was sufficient to inactivate the enzyme.