The electrophysiological effect of adenosine triphosphate (ATP) on the enzymatically dispersed smooth muscle cells from rat vas deferens was investigated. ATP always induced depolarization accompanied with a reduction in membrane resistance. In a whole cell voltage clamp experiment, an inward current was recorded when the cell was exposed to ATP-containing solution. The ATP-induced current disappeared within 2 min even in the continuous presence of ATP, which may indicate that the cells were desensitized to this compound. The ATP-induced current was also recorded in the cells superfused with 10(-5) M nicardipine or in the Cs-loaded cells, eliminating the possible involvement of voltage-gated Ca and K current. During cell-attached patch clamp, an elementary current having a mean conductance of 20 pS was observed when the intrapipette solution was changed to ATP-containing solution. The estimated zero current potentials of the ATP-activated macroscopic current and elementary current were about 0 mV. These results suggest that ATP exerts its transmitter-like action by activating ion channels in smooth muscles.