This experiment was designed to reveal the developing pattern of the ECG occurring in chick embryos during the period between 11 and 22 somites and to relate these findings to the appearance of blood circulation and the ability of the heart to exhibit atrioventricular block under the influence of digitalis. Using a unipolar electrode placed directly upon hearts of a graded series, the first recorded ECG was a simple V wave at 11 somites (sometimes 10) which increased in amplitude and rate through the 16 somite stage. In 16 or 17 somite embryos two deflections appeared on the descending limb of the V wave. In the light of subsequent changes these were recognized as the first indications of the P wave and QRS complex. Within a n hour of development (17 to 18 somites) three events coincided, i.e., appearance of the primitive P wave and QRS complex; the beginning of blood circulation as evidenced by flow through extraembryonic capillaries; and the ability of the hearts to show a block at the AV junction. This last occurs despite the fact that an obvious PR interval is not present at 18 somites. However, from the 18 to the 21 to 22 somite stage rapid changes in the pattern of the ECG were recorded. The PR interval emerges; the P wave and QRS complex become incisive and are followed by a n obvious T wave.The developmental pattern is the exact duplicate of the changes which can be recorded in reverse, by treating a single 60 hour embryonic heart with eserine (Paff, Boucek and Glander, '66) or apparently any acetylcholinesterase inhibitor (work in progress).