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1. A technique has been developed for the study of embryonic heart rate during the last week of incubation in a population of 84 embryos.
2. Heart rate is characteristic for a particular embryo but there are clear differences between individuals.
3. The population mean heart rate decreased from 262 beats/min at 14 d to a minimum of 250 beats/min at 19.5 d and then increased rapidly to more than 270 beats/min before hatching.
4. This pattern was repeatable and independent of the flock age.
5. Embryonic heart rate was also independent of egg size and hatching time.
6. The heart rate of male embryos was one to two beats/min lower than that of females.