This study examined the transduction pathways activated by epinepherine in the pacemaker region of the toad heart. Recordings of membrane potential, force, and intracellular Ca2+ concentration ([Ca2+]i) were made from arrested toad sinus venosus. Sympathetic nerve stimulation activated non-α-, non-β-adrenoceptors to evoke a membrane depolarization and a transient increase in [Ca2+]i. In contrast, the β-adrenoceptor agonist isoprenaline (10 μM) caused membrane hyperpolarization and decreased [Ca2+]i. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.5 mM) mimicked the isoprenaline-evoked membrane hyperpolarization. Epinephrine (10–50 μM) caused an initial membrane depolarization and an increase in [Ca2+]i followed by membrane hyperpolarization and decreased [Ca2+]i. The membrane depolarizations evoked by sympathetic nerve stimulation or epinephrine were abolished either by the phospholipase C inhibitor U-73122 (20 μM) or by the blocker ofd- myo-inositol 1,4,5,-trisphosphate-induced Ca2+ release, 2-aminoethoxydiphenyl borate (2-APB, 60 μM). Neither U-73122 nor 2-APB had an affect on the membrane hyperpolarization evoked by β-adrenoceptor activation. These results suggest that in the toad sinus venosus, two distinct transduction pathways can be activated by epinephrine to cause an increase in heart rate.