Young (3-days-old) embryonic chick hearts have slowly-rising spontaneous action potentials, dependent on tetrodotoxin-insensitive slow Na+ channels. When the hearts were placed into organ culture for 5-11 days, action potential duration was markedly increased by 260-370%, and a notch appeared between the initial spike phase and the plateau phase in some hearts. The spike amplitude was mainly dependent on [Na]0, whereas the plateau amplitude was dependent on [Ca]0. Thus, the young embryonic hearts develop slow Ca2+-Na+ channels (while retaining the slow Na+ channels) during organ culture, and the spike phase and the plateau phase of the slow action potentials are mainly dependent on currents through slow Na+ channels and through slow Ca2+-Na+ channels, respectively. The effects of Mn2+ (a specific blocker of slow Ca2+-Na+ channels) and verapamil (a blocker of slow Na+ channels as well as of slow Ca2+-Na+ channels) on the spike phase and the plateau phase were examined. Mn2+ (0.5 mM) and verapamil (5 microM) depressed the plateau duration and overshoot. Verapamil did not decrease the maximum rate of rise (Vmax), but Mn++ produced a small, but significant, decrease. High concentrations (10/30 microM) of verapamil depressed the action potential amplitude and Vmax, and abolished the spontaneous action potentials. These results indicate that slow Ca2+-Na+ channels appear de novo during organ culture of young embryonic hearts.