Developmental changes in the spontaneous action potential were measured using optical signals from early embryonic chick heart stained with a potential sensitive merocyanine-rhodanine dye. 2. The wave-length dependence of the dye signal is triphasic in early embryonic chick heart with a decrease in absorption from 525 to 600 nm, an increase from 625 to 720 nm, and a decrease at 750 nm. The signal was largest at 700 nm. 3. The magnitude of the spontaneous absorption signal increased as the development of the embryonic heart proceeded. In addition, the magnitude of the absorption signal differed among the various regions of an early embryonic chick heart and the number of electrically active cells increased dramatically throughout the 7-9 somite developmental stages. 4. The number of the electrically active cells was largest in the right portion of the ventricle at the 7-9 somite developmental stages. 5. The shape of spontaneous absorption signals could be classified into four types in the developmental stages between 7 and 9 somites: Signals resembling the cardiac action potential and the pace-maker potential were not recorded until the 8-9 somite developmental stage.
