The effects of 3,6-dimethylamino-dibenzopyriodonium formiate (IHC-64) on action potential and slow inward calcium current (I(si)) were investigated in isolated guinea pig ventricular myocytes using whole cell voltage clamp recording. IHC-64 (20 and 200 mumol/L) was shown to markedly decrease APD20 and APD90. However, in the presence of nisoldipine, no obvious effects of IHC-64 on APD20 could be detected. Under voltage clamp conditions, IHC-64 (20 and 200 mumol/L) caused a significant decrease in I(si). The results indicate that IHC-64 can block the calcium channels in isolated guinea pig ventricular myocytes.