I_f was shown to be present in adult human atrial and ventricular myocytes but data obtained from infant myocytes are lacking. We have studied I_f in isolated ventricular myocytes from children undergoing surgical correction of tetralogy of Fallot (TOF; n = 5; mean age: 15.3 months). All recordings were made with the patch clamp technique in the whole cell mode at a temperature of 36–37 °C. A modied Tyrode solution containing 25 mM KCl was used to amplify I_f. Considering I_f to be present when its current density at –120 mV was greater than 0.5 pA/pF, I_f could be found in 28 out of 32 myocytes (88%). The mean current density was –2.01 ± 0.3 pA/pF (mean ± S.E.M.). First current activation occurred at –70 mV and I_f could be reversibly inhibited by superfusing the myocytes with CsCl (2 mM). Half maximal activation (V_1/2) of I_f was at –80.3 ± 1.0 mV (n = 28). Beta-adrenergic receptor stimulation with isoproterenol (1 µM) caused an acceleration of current activation and a shift of V_1/2 by 7.88 ± 1.8 mV (n = 10) to less negative potentials. This study provides rst evidence that the hyperpolarization-activated pacemaker current I_f is present in infant human ventricular myocytes. Our results suggest that I_f in ventricle of infants suffering from TOF has similar properties as I_f in adult ventricle.