We show that ATP-sensitive K+ channels of excised inside-out membrane patches of rat ventricular myocytes show considerable variation in their sensitivity to ATP. In 102 different membrane patches IC50 values ranged from 9 to 580 microM ATP and Hill coefficients from 1% to 6.41% of patches showed openings of ATP-sensitive K+ channels in the presence of 1 mM ATP. These results considerably widen the range of internal ATP concentrations over which one might expect activation of the ATP-sensitive K+ current in cardiac myocytes.