Little information is available concerning the modulation of cardiac function by β-adrenergic agonists in early-stage embryonic mammalian heart. We have examined the effects of isoproterenol (Iso) on the spontaneous beating rate and action potential (AP) configuration in embryonic mouse hearts at 9.5 days postcoitum (dpc), just 1 day after they started to beat. Iso (3 μM) increased the spontaneous beating rate in whole hearts, dissected ventricles, and isolated ventricular myocytes. In ventricular myocytes, Iso also increased the slope of the pacemaker potential and the action potential duration but decreased the maximum upstroke velocity. In whole cell voltage-clamp experiments, the Ca2+-channel currents were measured as Ba2+currents ( IBa). In 9.5-dpc myocytes, IBawas enhanced significantly from −4.7 ± 0.9 to −6.7 ± 1.2 pA/pF (by 52.4 ± 14.8%, n = 10) after the application of Iso. Propranolol (3 μM) reversed the effect of Iso. Forskolin (For, 10 μM) produced an increase in IBaby 95.5 ± 18.8% ( n = 8). In ventricular myocytes at a late embryonic stage (18 dpc), 3 μM Iso caused an appreciably greater increase in IBafrom −6.2 ± 0.5 to −14.5 ± 2.2 pA/pF (by 137.8 ± 33.0%, n = 8), whereas the increase in IBaby 10 μM For (by 120.0 ± 23.0%, n = 7) was comparable to that observed in the early stage (9.5 dpc). These results indicate that the L-type Ca2+-channel currents are modulated by β-adrenergic receptors in the embryonic mouse heart as early as 9.5 dpc, probably via a cAMP-dependent pathway.