This study was undertaken to assess the risk of developing complete heart block during general anesthesia in patients with bundle branch block. His bundle electrograms were recorded during sinus rhythm and after atrail pacing in 11 patients (5 LBBB, 2 RBBB, 3 RBBB and LAH, 1 RBBB and LBH) before and after administration of Pentothal 0.20 g e.v., Succinylcholine 1 mg/kg e.v., and breathing of fluothane 1%or Ethrane. Minimal effects on sinus functions and A-V node conduction was observed during anesthesia; Fluothane only increased slightly AH intervals (+11%). Both Fluothane and Ethrane effects on HV conduction was insignificant. In 9 patients HV intervals increased of 5% after Succinylcholine; 2 patients developed a complete heart block distal to his after the drug. Possible causes of the complete heart block are discussed and a direct effect of Succinylcholine is hypothesized.