The effect of tanshinone II-A (TSN) on L-type Ca²⁺ current of single ventricular myocyte from guinea pig was studied by using whole cell voltage -clamp technique. Langendorff apparatus and method of recirculating digestion with proteinase (0.2 g·L^-1) were also used and, 70% to 80% isolated cells were rod-shaped Ca²⁺-tolerant myocytes with a clear striation. When the membrane potential was held at -40 mV, cell was depolarized to 0 mV for 250 ms at a frequency of 0.5 Hz. TSN 10, 20 and 40 μmol·L^-1 markedly reduced Ca²⁺ inward current by 35.2%, 57.7％ and 74.7％ respectively, without affecting the voltage-dependence of maximal Ca²⁺ current activation. After washing out TSN, the Ca²⁺ current partially recovered. In addition, TSN shortened the action potential duration in a concentration-dependent manner. These data indicate that the blocking effect of TSN on the activation of L-type calcium channels is voltage-independent.