As a contribution to the project "Development of endothermy of avian embryo" (DFG Tö 181-1) the effect of a short term hypothermia on the heart rate and the inner egg temperature of chick embryos of different ages was examined. The purposes was to verify the effect of manipulations on the embryo outside of the incubator. The physilogical parameters of the heart rate (HR) and inner egg temperatur (IET) served as measurements. As the temperature regulation, among others, is dependent upon cardiovascular conditions, the HR was capable of being a sensitively reacting and easily measurable parameter to determine the reactionary ability of the embryo. The IET reflects the influence of the ambient temperature as well as the influence of the heat production on the embryo. It is thereby well suited to a complete estimation of the response to thermal stimulation. To detect the HR, a semiinvasive method of the electrocardiogram charting was used. In order to mesure the IET, an invasive methode was developed, with which the temperatur near the embryo could be measured over a long time, without substantially influencing the embryo. The response of the HR and the IET of chick embryos at the 14th, 17th and 20th day of incubation during the cooling of the room temperature (21 °C) from 5 to 25 minutes was examined. A comparison between egg with live embryos and egg with dead embryos underlines the influence of blood circulation and metabolism on the IET. The resulting decline of the HR and IET during the cooling of the room temperature was dependent on the duration and the age of the embryos. During the rewarming under incubation conditions (37,4 °C), the HR and IET rose logarithmic. During a two hour rewarming, 86% of the examined embryos reached their original HR level and 64% reached their initial IET again. It can therefore be concluded that the adaptation time for embryos after treatment should be least two, if not four hours. The high linear correlation between HR and IET during the cooling and during the rewarming in all three age stage permit the conclusion from one parameter to the others and makes it possible to narrow the scope of future experiments.