 Dear students, in this topic, we shall discuss homeotherms and pykelo-therms, that is, warm-blooded and cold-blooded animals. Dear students, based on the stability of body temperatures of animals, the animals are traditionally classified into two groups, that is, homeotherms, commonly are traditionally known as warm-blooded animals and pykelo-therms, which are known as cold-blooded animals. The homeotherms are warm-blooded animals are able to regulate their body temperatures within a narrow physiological range. They have control on the mechanisms of heat production and heat loss. So they are able to maintain their body temperatures at a constant or nearly constant level in spite of large variations in ambient, that is, environmental temperatures. The animals that are traditionally included are mammals or birds. Mammals maintain a core body temperature between the ranges of 37 to 38 degree centigrade, while birds maintain their core body temperatures at nearly 40 degree centigrade. In contrast to homeotherms, pykelo-therms are cold-blooded animals lack the ability to generate high levels of heat production. Their body temperatures tend to fluctuate with the environmental or ambient temperatures. Cold-blooded animals include all kinds of fishes, amphibians, reptiles and other invertebrates. Dear students, the term used in the traditional classification of animals were actually used by early comparative physiologists. These early comparative physiologists included all invertebrates, fishes, amphibians and reptiles as cold-blooded or pykelo-thermic animals. They included all birds and mammals as warm-blooded or homeothermic animals. However, later research proved that these terms are not physiologically correct because animals' thermal behaviours vary in different times. Many pykelo-therms or cold-blooded animals are such that their bodies become warm-up and many endotherms are such that the temperature of some of the regions of their body reaches a very low level or they become cold-blooded. Similarly, they vary their body temperatures seasonally in different times and become comparatively colder. It means that both terms are not fixed and animals' behaviours are not completely warm-blooded or cold-blooded or homeothermic, i.e. to maintain a temperature level for a long time. That is why use of these terms in modern scientific physiological literature has been abundant.