Ambient temperature has an impact on the energetic requirements of all animals, causing changes in oxygen consumption and heart rate. Considering the impact that external factors have on adjusting physiological functions, it is important to know how natural alterations in temperature affect the body. The objective of this study was to examine the change in heart rate and oxygen consumption of Red Junglefowl (Gallus gallus) when exposed to naturally occurring low temperatures and if there was a difference in these parameters between the sexes. Furthermore, there was an interest in investigating the occurrence of circadian rhythms in heart rate. Heart rate was measured for indoor-kept animals, exposed to a constant temperature of 20 °C, and outdoor-kept animals that were subjected to variations in ambient temperature. Exposing the chickens to low ambient temperatures, oxygen consumption was monitored using a respirometer while simultaneously measuring their body temperature. An elevated heart rate could be observed during the day. No difference in heart rate was found between males and females, but oxygen consumption was higher for females and body temperature was higher for males. An inverse, linear relationship occurred between heart rate and oxygen consumption in relation to ambient temperature, causing them to increase in response to a decreasing temperature. These results indicate that the energy demand rises when animals are exposed to cold environments, followed by an elevation in oxygen consumption and heart rate in order to supply the body with the energy needed to maintain a stable body temperature.