The whole-cell current clamp and voltage clamp techniques were used to record the slow Na+ action potentials (APs) and the inward current in cultured single ventricular cells isolated from young (3 day-old) embryonic chicks. The slow Na+ APs had a +Vmax of 21.5 +/- 7.5 V/s (in 10 different single cells) and the macroscopic inward current responsible for the rising phase of these APs was a fast transient (ft) current. The ft inward current was sensitive to changes in [Na]o but not to changes in [Ca]o. This channel was found to be permeable to Li+ and Ba2+. Analysis of Na+ current decay suggests a second-order process of current decay. The slow Na+ APs and the ft inward current were insensitive to tetrodotoxin (TTX) and Mn2+. This current was also insensitive to the inorganic Ca2+ blockers, Ni2+, Cd2+ and La3+. At low concentration (10(-9)-10(-6) M) the organic Ca2+ blockers, (-)D888 and nifedipine had no effect on the TTX- and Mn2(+)-insensitive INa. However, at a high concentration (10(-5) M), the Ca2+ blockers, (-)D888 and nifedipine, completely blocked the slow Na+ APs and the TTX- and Mn2(+)-insensitive ft inward Na+ current responsible for the rising phase of the APs. High concentration of verapamil (10(-5) M) and D-600 (10(-5) M) had little depressant effects due to their frequency dependence. Apamin, a toxin in the bee venom, that was previously reported by our group to block the slow Ca2+ APs (Bkaily et al., 1985) and the slow Ca2+ current (Bkaily et al., 1988b), greatly decreased the TTX- and Mn2(+)-insensitive ft INa at 10(-10) M. Thus, the inward current responsible for the rising phase of the slow Na+ APs in 3 day-old embryonic chick heart shows fast transient activation and is TTX- and Mn2(+)-insensitive. This channel is highly sensitive to apamin and shares few characteristics with the Ca2+ channel and the TTX-sensitive fast Na+ channel.