PURPOSE: This study was performed to determine the effects of phytoestrogen on seminal vesicle excitability. MATERIALS AND METHODS: Single smooth muscle cells of seminal vesicle were obtained from rabbits using proteolytic enzymes(collagenase). Using single cell and channel recording methods of patch clamp, the various currents of potassium channels in smooth muscle cells were recorded. Potassium currents were divided into calcium dependent and independent. RESULTS: Most of the calcium dependent K currents were maxi-K currents and most of calcium independent ones were delayed rectifier K currents. Inside-out patch clamp technique was used to characterize the maxi-K channel. The channel showed outward rectification and calcium dependency. The single-channel conductance of this channel estimated from slope conductance was 119.4+/-11.7 pS under physiological conditions. These characteristics were typical properties of maxi-K channels. Application of genistein(10micronM) rarely affected the delayed rectifier K channel activities, but it evoked significant increase of maxi-K channel activities at both single cell and channel levels. CONCLUSIONS: From these results it is strongly suggested that the excitability and contractility of seminal vesicle might be modulated by genistein through a mechanism of maxi-K channel activation.