Tetrandrine, an alkaloid isolated from the Chinese herb, Radix stephaniae tetrandrae, has been used clinically as a hypotensive agent for a long time. Recently, several studies have demonstrated that tetrandrine behaves like a calcium entry blocker. In the present investigation, the whole cell version of the patch clamp technique was used to study the effect of tetrandrine on the type I (transient inward) calcium current in neuroblastoma cells. These results showed that tetrandrine inhibited the transient inward current, without affecting the channel kinetics. The effects of tetrandrine were dose-dependent and reversible but did not depend on the frequency of stimulation (use-dependence) or the membrane potential. These data clearly demonstrate that tetrandrine is a novel and potent antagonist of the transient inward current in neuroblastoma cells.