The effects of halothane, isoflurane, and enflurane on proximal (false tendon) and distal (apical) canine Purkinje fibers were measured in vitro to assess their comparative effects on fibers exhibiting characteristically long (proximal) and short (distal) action potential duration. High- and low-dose anesthetic effects were evaluated in three groups of six left ventricular preparations and were compared with the changes occurring at identical times in six control preparations using analysis of variance with repeated measures. Under control conditions in all groups, the action potential duration, measured at 90% repolarization (APD90, mean +/- SEM), of proximal fibers was longer than that of distal fibers (320 +/- 16 vs 252 +/- 11 ms, P less than or equal to 0.01). Halothane (0.3 and 0.7 mM), isoflurane (0.4 and 0.8 mM), and enflurane (0.6 and 1.0 mM) produced a dose-dependent decrease (P less than or equal to 0.01) of proximal fiber APD90 with less (P less than or equal to 0.01) change of distal fiber APD90 and reduced (P less than or equal to 0.05) regional differences of APD90 at the higher dose. The decreases of proximal fiber APD90 were greater (P less than or equal to 0.01) for 1.0 mM (1.7 MAC) enflurane (-66 +/- 7 ms) and 0.8 mM (3.0 MAC) isoflurane (-69 +/- 9 ms) than for 0.7 mM (2.9 MAC) halothane (-33 +/- 8 ms). We conclude that the regional actions of anesthetics on Purkinje fiber repolarization may influence conduction during the relative refractory period and the occurrence of arrhythmias associated with disparity of regional refractory characteristics in the ventricular conduction system.