The effect of halothane, tetrodotoxin (TTX), veratridine (VTD), and alterations of extracellular calcium ion concentration ([Ca++]0) on regional difference of canine Purkinje fiber action potential duration (APD50and APD90) were investigatedin vitroat a paced rate of 75 beats per min. Under control conditions (n = 15 heart) APD90of proximal (false tendon) fibers (289 ± 6 ms) always exceeded (P< 0.01) that of distal (apical) fibers (213 ± 4 ms). Halothane (0.35–1.07 mM) reduced regional differences of APD90by producing dose-dependent decreases of proximal APD90without decreases of distal APD90. The regional actions of halothane were similar to those of low (0.33–1.0μM) concentrations of the Na+channel antagonist TTX, which also decreased proximal APD90more than distal APD90. The actions of halothane in combination with TTX further decreased proximal APD90, whereas the Na+channel agonist VTD, which increased proximal APD90more than distal APD90, reversed the regional actions of halothane. Decreasing Ca++influx by reducing [Ca++]0from 1.8 to 0.6 mM increased proximal APD90more than distal APD90in a manner opposite to the regional actions of halothane. Although there was no difference between the values of APD90obtained for each region in the presence of halothane at 0.6, 1.8, and 3.6 mm [Ca++]0, the action of halothane decreasing APD90of proximal fibers was more prominent at 0.6 mM [Ca++]0because of the increased APD90of fibers under this condition. The findings are consistent with, but do not definitively prove, the hypothesis that halothane may decrease APD90of proximal Purkinje fibers by a mechanism similar to that of TTX involving inhibition of plateauphase inward Na+current.