The cardiac 'pacemaker' current is recorded in isolated sino-atrial node cells during hyperpolarizations at voltages from -40/-50 mV to -100/-110 mV, which corresponds to the range where diastolic depolarization occurs. if is a hyperpolarizing-activated current, carried by Na and K, and in the pacemaker voltage range is inward. These properties allow if to serve as a tool to generate and control the 'pacemaker' depolarization phase of the action potential in sino-atrial node cells. The current if has long been shown to mediate the accelerating action of catecholamines in the heart. More surprisingly, recent experiments show that low doses of acetylcholine exert on if a strong inhibitory action. This new finding modifies the view that the slowing of pacemaker activity caused by acetylcholine is essentially due to activation of a K-current.