To record the fast Na+ current, spheroidal heart cells enzymatically-dispersed from 3 approximately 18-day-old chick embryos were used for voltage clamping. The peak of currents in response to voltage steps of 200 ms long from holding potentials of -90 approximately -105 mV were measured. The current-voltage curves for the peak inward current showed U-shaped relations; the averaged peak current of about -1400 pA was observed at about -30 mV and the current reversed sign at +40 approximately +50 mV. Both the peak current and the reversal potential values showed marked [Na]o-dependence, i.e. reduced by 36% and by 20 mV, respectively, for a halved [Na]o. Tetrodotoxin (TTX) partially (10(-6) M) or completely (10(-5) M) suppressed the current. The steady-state inactivation of the current (h infinity) was characterized by the half inactivation voltage of around -80 mV and the slope factor of -4 approximately -8 mV. The half activation voltage and the slope factor for the steady-state activation (m infinity) were -55 mV and 4-6 mV, respectively. The electrophysiological and pharmacological properties were similar between young (3-day-old) and old (15-18-day-old) embryonic heart cells, excepting the much smaller current and the slower onset of TTX action in young embryonic hearts.