Block of sodium current by chlorpromazine in single ventricular myocytes isolated from guinea pigs was studied using the whole cell patch clamp technique. Chlorpromazine in micromolar concentrations reduced the amplitude of peak sodium current associated with step depolarizations from a holding potential of -140 mV. Concentration-response curves obtained with a holding potential of -140 mV were best fit by a 2:1 stoichiometry, and were shifted in the direction of lower concentrations when a holding potential of -100 mV was used. In agreement with this observation, the steady-state inactivation curve was shifted to more negative potentials by chlorpromazine. The block was not associated with any change in the time course of sodium current activation or inactivation during a depolarizing step. Chlorpromazine also produced marked use-dependent block as demonstrated by a cumulative increase in the block during a train of depolarizing pulses. This use dependence was due to a higher affinity of chlorpromazine for the inactivated state of sodium channels than for the resting state and to a very slow repriming of the drug-bound sodium channels from inactivation. These blocking actions could contribute to the antiarrhythmic effects of chlorpromazine at low concentrations and to the cardiotoxic effects at high concentrations.