The effects of halothane and isoflurane on ventricular intracellular action potentials recorded in situ in pentobarbital anesthetized rats were studied. Halothane (0.5, 1 and 2 vol.%) and isoflurane (0.75, 1.5 and 3 vol.%) did not have identical effects on rat epicardial action potentials recorded by floating microelectrodes. However, over the concentration range tested, both anesthetics reduced blood pressure and heart rate to a similar extent. Isoflurane did not effect the maximum rate of rise of the action potential amplitude. However, 3 vol.% isoflurane reduced the resting membrane potential from -72+/-2 to -65+/-3 mV (mean+/-SEM, p<0.05) while the highest concentration of halothane had no effect. Halothane (2 vol.%) reduced action potential amplitude from 74+/-4 to 65+/-3 mV (p<0.05) and reduced the maximum rise rate of action potential from 175+/-21 to 133+/-8 V/s (p<0.05). Both isoflurane and halothane prolonged action potential duration at 10, 25 and 50% repolarization while only halothane significantly shortened action potential duration at 75% repolarization, Thus the effects of halothane and isoflurane on ventricular transmembrane action potentials were similar, but not identical. The relevance of such observations to the antiarrhythmic actions of halothane, but not isoflurane in this species is not clear.