Mechanical stimuli to the cardiac myocytes initiate many biochemical and physiological events. Stretch-activated cation channels have been suggested to mediate these events. In this study, cell-attached and inside-out excised-patch clamp methods were used to identify stretch-activated cation channels in adult rat atrial myocytes. Channel openings were increased in cell-attached configuration when negative pressure was applied to the pipette, and also in inside-out excised patches by negative pressure. The channel was not permeable to Cl⁣, Na⁢ and Cs⁢, but selectively permeable to K⁢, and the degree of activation was dependent on the magnitude of negative pressure (full activation at ∼ ⁣50 mmHg). In symmetrical 140 mM KCl, the slope conductance was 51.2⁑3 pS between the potentials of ⁣80 and 0 mV and 55⁑6 pS between 0 and ⁢80 mV (n=5). Glibenclamide (100μM) or ATP (2 mM) failed to block the channel openings, indicating that it is not ATP-sensitive K⁢ channel. Arachidonic acid (30μM), which has been shown to activate a K⁢ channel cooperatively with membrane stretch, did not affect the channel activity. GdCl3 (100μM) also did not alter the activity. These results demonstrate that the mechanical stretch in rat atrial myocytes activates a novel K⁢-selective cation channel, which is not associated with other K⁢ channels such as ATP-sensitive and arachidonic acid-activated K⁢ channel.