Modulation by intracellular Ca 2+ concentration ([Ca 2+] i) of the reversal potential for the fast Na + current (I Na) in 17-day-old embryonic chick ventricular cardiomyocytes was examined using a whole-cell voltage-clamp technique. Experiments were performed at room temperature (22°C). Test pulses were applied between −60 to +50 mV from a holding potential of −90 mV. The I Na was TTX-sensitive, and the reversal potential was +47.3 ± 2.2 mV n = 20 at pCa 10 . Neither pCa 7 or pCa 10 caused any effect on the peak amplitudes of I Na, but the reversal potential at pCa 7 shifted in the hyperpolarizing direction by 10.5 ± 2.8 mV n = 10, p < 0.05 , as compared with that at pCa 10. The hyperpolarizing shift was also observed by application of taurine, and behaved in a concentration-dependent manner; by 10.4 ± 2.3 mV n = 8, p < 0.05 at 10 mM , and by 12.1 ± 2.3 mV n = 8, p < 0.05 at 20 mM. Even when taurine at low concentration (5 mM) enhanced I Na, the similar shift of the reversal potential occurred. These results suggest that the shift of reversal potential of the I Na in embryonic chick cells would be involved with somewhat cellular mechanism dependent on [Ca 2+] i, which may play an important role for cardiac functions of the embryonic cells.