The fertilized chicken egg (embryonic chicken) shares the basic mechanism of electrophysiology and ionic currents with mammalian hearts. However, little is known about the cardiac repolarization process during embryo growth. We examined if the electrocardiographic QT interval was dependent on the stage of embryonic development. ECGs were recorded from 4 fertilized chicken eggs for 6 days (day 13 to day 18). Eggs were kept in an incubator with a temperature of 37.4°C. Chronic electrodes were implanted and a 5-minute ECG was recorded each day. Three measurements were made in noise free segments: Heart rate, QT interval, and the Fridericia corrected QT interval (QTcF), and an average value was calculated. The QTcF interval decreased during embryo development. On day 18, QTcF was shorter than QTcF on day 13: 341 ms (95% CI: 332 ms to 349 ms) versus 417 (95% CI: 325 ms to 509 ms), p=0.037. In fertilized chicken eggs, the repolarization process changes during development. This property makes the chicken embryo an interesting model for the study of drug effects on the QT interval at different stages of embryo development.