Pace-maker type action potentials and rhythm generation in very early embryonic chick hearts have been monitored using a voltage-sensitive merocyanine-rhodanine dye. 2. Rhythmicity in recurrence of the spontaneous action potentials was evident at the 7 somite developmental stage, and the rhythm was completely organized by the early period of the 9 somite stage; just before the first contraction. The rhythmic recurrence of action potentials was increased in frequency as development proceeded from the 7 to the 9 somite stage, and presumably gives rise to the rhythm of the initial contractions. 3. Optical signals resembling the pace-maker type action potential with a diastolic depolarization phase were first detected in embryonic hearts at the 8 somite stage. At this stage, pace-maker type action potentials were detected from various regions, such as the ventricle and the unfused primordia at the atrium level. 4. Regionalization of pace-maker type action signals was exhibited at the early period of the 9 somite stage. At this stage, the pace-maker type signals were often evident at the atrium level, while the cardiac type signals were detected in the ventricular region. 5. Hence it is concluded (i) that the rhythmicity has already been generated at the 7 somite developmental stage, (ii) that the pace-making cells are widely distributed in the embryonic hearts at the 7-8 somite stages and (iii) that the appearance of the pace-maker potential is initially localized to the atrium level at about the 9-10 somite stages.
