The effect of lidocaine on His-Purkinje conduction in dogs with ischemic damage to the His bundle was compared with the effect of lidocaine in normal dogs. The anterior septal artery was ligated in 14 dogs, and 30 minutes later atrial pacing was performed to increase residual ischemic damage. Four to 6 days later, His bundle recordings were obtained during sinus rhythm and atrial pacing before and after the administration of lidocaine in a dose of 2 mg/kg and a total dose of 4 mg/kg. His bundle recordings were also obtained in nine control animals beofre and after the administration of lidocaine. Lidocaine significantly increased the H-V time in the animals with ischemic damage during sinus rhythm and at all packing rates. It also resulted in advanced His-Purkinje conduction defects including His bundle block and right bundle branch block in these animals. In contrast, the effect of lidocaine in the normal animals was negligible. It is concluded that lidocaine significantly depresses His-Purkinje conduction in the setting of preexisting ischemic damage. These results suggest that lidocaine may be used as a diagnostic tool to unmask latent His-Purkinje conduction defects due to ischemia.