Objective: We have investigated the properties of the hyperpolarization-activated (I f ) current in pacemaker cells from the mouse sino-atrial node (SAN). Methods: The I f current was studied in cells isolated enzymatically from the SAN region of adult C57BL6/J mice. The whole-cell variation of the patch-clamp technique was employed to investigate the basic properties of I f . Results: In mouse SAN cells, the I f current density at -120 mV was 18+/-2 pA/pF (n=23). I f was not detected in cells showing atrial-like morphology that were also found in SAN preparations (n=7). I f was blocked by 5 mM Cs + , was inhibited by application of 5 μM acetylcholine, and was increased by 10 μM noradrenaline. The I f current reversal potential was -31+/-2 mV under physiological concentration of Na + and K + ions. Lowering the extracellular Na + concentration reduced I f amplitude, while increased when the extracellular K + concentration was augmented. I f voltage for half activation was -87+/-1 mV (n=6). Conclusions: We conclude that the native I f current in mouse SAN cells shows functional properties that are similar to I f described in rabbit SAN tissue. This study opens the possibility of investigating the involvement of I f in the regulation of heart rate in genetically modified mice.