1. Effects of imipramine and haloperidol on voltage-gated sodium channels were investigated in guinea-pig isolated ventricular myocytes by the whole-cell patch clamp technique. Some additional experiments were also performed with chlorpromazine for the purpose of comparison. 2. All test drugs in micromolar concentrations suppressed the amplitude of peak sodium current associated with step depolarization from a holding potential of -140 mV in a reversible manner. The order of potency was chlorpromazine greater than imipramine greater than haloperidol. 3. Dose-response curves obtained with a holding potential of -140 mV were best fitted by 2:1 stoichiometry in all three drugs and were shifted in the direction of lower concentrations when a holding potential of -90 mV was used. 4. The drug-induced block was not associated with any change in the time courses of sodium current activation and inactivation. 5. Steady-state sodium channel inactivation curve was shifted in the direction of more negative potentials by the drugs. 6. All three drugs also produced marked use-dependent block as demonstrated by a cumulative increase in the block during a train of depolarizing pulses. 7. The use dependence was due to a higher affinity of the drugs for the inactivated state of sodium channels than the resting state and to a very slow repriming of the drug-bound sodium channels from inactivation. 8. The steady-state and use-dependent block of voltage-gated sodium channels by psychotropic drugs may contribute to their cardiotoxic and perhaps antiarrhythmic effect.