In order to elucidate the properties of the background current whole cell patch clamp studies were performed in rabbit ventricular cells. Ramp pulses of ±80 mV from holding potential of 40 mV(or 20 mV) at the speed of 0.8 V/sec were given every 30 sec(or 10 sec) and current-voltage diagrams(I-V curve) were obtained. For the activation of the background current isoprenaline, adenosine 3 ,5 -cyclic monophosphate(dBcAMP), guanosine 3 ,5 -cyclic monophosphate(cGMP), and N⁶-2 -o-dibutyryladenosine 3 ,5 -cyclic monophosphate(dBcAMP) were applied after all known current systems were blocked with 2mM Ba, 1 mM Cd ,5 mM Ni, 10 μM diltiazem, 10 μm ouabain, and 20 mM tetraethylammonium(TEA). The conductance of background current in control was 0.65±0.69 nS at 0 mV, its I-V curves was almost linear and reversed near 50 mV. When there was no taurine in pipette solution, isoprenaline hardly activated the background current but when taurine existed in pipette solution, isoprenaline activated the larger background current. Cyclic AMP or cyclic GMP alone had little effect on the activation of the background current, while cGMP potentiated cGMP effect. When the background current was activated with cGMP and cAMP, isoprenaline could not further increased the background current. The background current activated by isoprenaline depended on extracellular Cl^- concentration and its reversal potential was shifted according to chloride equilibrium potential. The change of extracellular Na⁺ concentration had little effect on reversal potential of the background current activated by isoprenaline.