Using the whole-cell voltage-clamp technique, early embryonic tetrodotoxin (TTX) and Mn(2+)-insensitive slow Na+ current was detected in 10-22 week old fetal human heart cells as well as in 1 day old and young cardiomyopathic hamster myocytes. This slow Na+ current in both heart cell preparations has the same kinetics and pharmacology. This type of slow Na+ current was absent in heart cells of newborn and young normal hamsters and became less present in myocytes of 19 and 22 week old human heart myocytes. Our results demonstrate that the slow Na+ channel does exist in early fetal human life and this type of channel continues to be functional after birth in myocytes of the hereditary cardiomyopathic hamster.