The effects of 1.2, 1.7, and 2.3 MAC enflurane (ENF), halothane (HAL), and isoflurane (ISO) on specialized atrioventricular (AV) conduction times were compared with awake (control) in 23 dogs that were chronically instrumented for His bundle studies. Compared with awake, 1.2 MAC ENF and HAL produced 17% and 18% increases in AV nodal conduction time, respectively. There was little added prolongation related to depth of ENF or HAL. ISO did not prolong AV nodal conduction time at 1.2 MAC compared with awake, but it did prolong conduction compared with awake at 1.7 (9%) and 2.3 MAC (12%). All agents produced an approximate 5% increase in His-Purkinje and ventricular conduction times compared with awake, with little additional effect related to depth of anesthesia. In separate experiments in ten of these dogs, anesthetic effects on conduction were determined following combined autonomic blockade with atropine and propranolol. During autonomic blockade, there was no effect of any anesthetic compared with awake, or to increased level of anesthesia, on specialized AV conduction times. The authors conclude that of the major inhalation anesthetics in current clinical use, ISO is least depressant of and ENF and HAL about equally depressant of AV nodal and His-Purkinje conduction times. Furthermore, depression of AV nodal conduction appears to be an indirect rather than direct effect of anesthesia. Finally, most depression of conduction occurs with light anesthesia, with little added depression related to depth of anesthesia over levels likely to be encountered clinically.