&NA;Pinacidil belongs to a novel group of compounds that enhance the potassium permeability of vascular smooth muscle. Evidence also exists that this drug enhances the potassium permeability of cardiac tissue. The purpose of the present investigation was to determine if pinacidil alters potassium‐channel activity in heart and, if so, which potassium channel is the target. We used the whole‐cell arrangement of the patch voltage clamp to record membrane currents from isolated guinea pig ventricular cells. In solutions designed to isolate potassium currents, pinacidil enhances a time‐independent current positive to the potassium equilibrium potential. Current measured at voltages negative to the potassium equilibrium potential are essentially unaltered by the drug. The potassium sensitivity of outward current indicates that the target for the drug is a potassium channel. Experiments designed to test for voltage‐dependent channel gating strongly suggest that the pinacidil‐sensitive current is not voltage gated. Pinacidil‐sensitive current is blocked by externally applied Ba2+, Cs+, and tetraethylammonium ion. In addition, it is potently blocked after external application of 100 nM glibenclamide. Taken along with the time‐ and voltage‐independent properties of pinacidil‐sensitive current, this pharmacology strongly suggests that the target for pinacidil in heart is the ATP‐sensitive potassium channel. (Circulation Research1989;65:436‐445)