Cellular electrophysiological remodeling of the infarcted heart may lead to the deterioration of cardiac function and/or to arrhythmias. The present study was designed to characterize the functional expression of the hyperpolarization-activated current (I_f) and its modulation by β₁-, β₂- and β₃-adrenoceptor (AR) subtypes, in patch-clamped ventricular myocytes isolated from the heart of post-myocardial infarcted (PMI) rats and sham-operated control (SHAM) rats. Maximum specific conductance of I_f was significantly higher in left ventricular myocytes (LVM) from PMI rats compared to right ventricular myocytes from PMI rats as well as LVM and RVM from SHAM rats. All other basic properties of I_f were similar. β₁AR stimulation with noradrenaline caused a rightward shift of V_H in LVM from PMI rats which was significantly smaller (52.2%) than in LVM from SHAM rats. Incubation with pertussis toxin (PTX) largely restored the effect of β₁AR in PMI cells (86.6% vs. SHAM cells), but did not affect β₁AR response in SHAM cells. β₂AR response was significantly and equally increased by PTX-pretreatment (by 94% in SHAM and 87% in PMI cells). Conversely, β₃AR stimulation by the selective agonist SR 58611A caused a leftward shift of the activation curve which was significantly larger in PMI cells than in SHAM cells (P<0.01). β₃AR response was blunted by PTX-pretreatment, by incubation with N^G-monomethyl-l-arginine acetate or by the selective β₃AR antagonist SR 59230A 1 μM. In conclusion, I_f is significantly overexpressed in LVM from PMI rat hearts. In these cells, I_f modulation by β₁AR is significantly depressed while β₃AR modulation is markedly enhanced, probably reflecting the increased activity of PTX-sensitive G_i proteins in PMI cells.