Dopamine shifted the voltage-dependence of the slow outward current that underlies spontaneous diastolic depolarization to a more positive voltage range and increased the rate constants of inactivation of this current at voltages negative to -80 mV. Dopamine increased the magnitude of the slow inward current and the delayed outward current. Dopamine had an inconsistent effect on the positive dynamic current and the time-independent potassium current. These effects were consistent with effects on the slope of diastolic depolarization, plateau voltage, and action potential duration and are similar to those of epinephrine. The effects of dopamine on the slow inward current and on the delayed outward current were abolished by verapamil, a slow channel blocker. This finding substantiates that outward potassium currents are modulated by intracellular calcium.