Objective: I _f is a hyperpolarization-activated current, which plays a key role in determining the spontaneous rate of cardiac pacemaker cells. We have previously shown that I_f is also expressed in left ventricular myocytes isolated from spontaneously hypertensive rats; in these cells, its occurrence and density is linearly related with the severity of myocardial hypertrophy. Since hypertrophy induces a re-expression of genes encoding fetal proteins, we investigated changes in I_f properties during post-natal development. Methods: Fresh ventricular myocytes were enzymatically isolated from the heart of 1–2- to 28-day-old Wistar rats. The whole-cell configuration of the patch-clamp technique was employed to record the action potential and I_f. Results: Membrane capacitance, an index of cell size, progressively increased from 13±1 pF at 1–2 days to 66±4 pF at 28 days of age (p<0.01). At 1–2 days, a cesium-sensitive hyperpolarization-activated inward current (I_f) was recorded in the majority of tested cells (n=51). The midpoint of the activation curve (V_1/2) was −78±2 mV (n=32), and specific current conductance of fully activated I_f (g_f,max) was 60±11 pS/pF. Reversal potential (V_rev) measured by tail-current analysis was −24±3 mV (n=8). Reduction of extracellular Na⁺ from 140 to 35 mM or extracellular K⁺ from 25 to 5.4 mM caused a shift of −12±1 mV (n=3) or −11±2 mV (n=5) of V_rev, respectively. Occurrence of I_f decreased with aging, being present in 64%, 48% and 32% of cells at 10, 15 and 28 days, respectively. When present, I_f density was significantly smaller than at 1–2 days (p<0.05), reaching a value of 8±2 pS/pF at 28 days. However, V_1/2 did not change in the older rats, being −80±2, −83±4 and −85±3 mV at 10, 15 and 28 days, respectively. V_rev at 10 and 15 days was −27 and −28 mV, respectively, thus suggesting that channel selectivity did not change. Conclusions: The pacemaker current, I_f, is expressed in ventricular myocytes from neonatal rats and progressively disappears; when present, it shows electrophysiological properties similar to I_f re-expressed in hypertrophied adult rat ventricular myocytes. Thus, it is likely that the occurrence of I_f in ventricular myocytes of hypertrophied and failing hearts is due to the re-expression of a fetal gene.