Adenosine triphosphate (ATP) depolarized the membrane of bullfrog sympathetic ganglion cells by decreasing resting K+ conductance. ATP also depressed the maximum amplitude of after-hyperpolarization of action potentials. Voltage-clamp study revealed that ATP markedly suppressed the TEA-insensitive K+ current which appeared to correspond to the M-current, while it affected less significantly on the delayed rectifier K+ current. It was suggested that ATP depolarized resting membrane by suppressing resting K+ conductances, including the M-current, and also depressed the after-hyperpolarization of action potentials by suppressing both the M-current and delayed rectifier K+ current.