Calcium channel currents of bovine chromaffin cells are blocked by the bis-benzyl-isoquinoline alkaloid tetrandrine, which is isolated from the roots of the Chinese medical herb Stephania tetrandra. The half-maximal inhibition (IC50) occurred at a concentration of about 10 μM. Bovine chromaffin cells possess no low-voltage-activated, but different types of high-voltage activated calcium channel currents. Tetrandrine blocks omega-conotoxin-sensitive N-type, dihydropyridine-sensitive L-type and omega-conotoxin- and dihy-dropyridine-insensitive calcium channel currents of chromaffin cells. The I/V relationship is not shifted by tetrandrine, indicating that the block is not voltage-dependent. After tetrandrine application the currents become transient in a depolarization- and a concentration-dependent manner. Tetrandrine blocks open calcium channels and the block is use-dependent. When the holding potential returns to resting levels, the channels are unblocked in about 1 sec. © 1994.