The role of IK (delayed rectifier current) and If (hyperpolarization-activated current) in dominant and subsidiary pacemaker ranges was studied in single myocytes isolated from the guinea pig sino-atrial node by means of a perforated patch-clamp technique. In the dominant pacemaker range (approx. –55 to –40 mV), IK tails are present whereas If is not activated. In the subsidiary pacemaker range (approx. –80 to –70 mV), If is large whereas IK is minimal and reversing. The threshold for If activation is more negative at short time intervals. Larger or longer depolarizations to –40 mV and +20 mV deactivate If more and are followed by faster reactivation of If. Steps of 200–300 ms duration to +20 mV completely deactivate If. The slope conductance decreases during depolarizations at –40 and +20 mV and quickly re-increases after the steps. The If deactivation range is between –70 and +10 mV, with a V1/2 of –35 mV. Depolarizations from –80 to +20 mV at a rate of 120/min limit the subsequent If reactivation owing to the short diastole. We conclude that IK plays a predominant role in the dominant pacemaker range and If does so in the subsidiary pacemaker range. Either pacemaker mechanism is used by sino-atrial node cells depending on the diastolic potential range. A previous depolarization markedly increases the amplitude and rate of If reactivation.Key words: dominant and subsidiary pacemaker ranges, sinus discharge and dual pacemaker mechanisms, delayed rectifier current IK, hyperpolarization-activated current If, factors regulating If, pacemaker shifts.