Based on evidence showing that instantaneous heart rate (IHR) of chick hatchlings responds to exposure to altered ambient temperature (Ta) (27), we elucidate here the developmental timeline for the homeothermic response of HR in newly hatched chicks (days 0-7) maintained at room temperature (about 24-27°C). Hatchlings were exposed to Ta of 25°C, 35°C and 25°C for 1-h periods, respectively, and IHR was measured together with skin temperature (Ts) during this warming and cooling bout. Early 0-day-old (0-d) chicks responded to warming and cooling exposures with various changes in HR baseline. In newly hatched chicks (0-7 hrs old), HR baseline was elevated during warming 126 bpm, N=13) and declined during cooling (-94 bpm). With progress of development on day 0, the elevation of HR baseline during warming decreased and advanced 0-d chicks tended to decrease HR baseline during warming rather than increase HR. The more developed 1-d to 7-d chicks exhibited the expected homeothermic decrease in HR during warming. The diurnal variations of HR responses during warming and cooling on the first day of post-egg life indicate that pronounced development of thermoregulatory competence occurs during the day of hatching (day 0). The response of IHR fluctuations to altered Ta was observed in the form of low and high frequency oscillations. High frequency oscillations corresponding to respiratory sinus arrhythmia developed as the hatchlings aged. There was a significant increase in the number of chicks exhibiting both low and high frequency oscillations and that depended on age and the development of thermoregulatory competence of hatchlings.