Metabolic rate is generally understood as the rate of heat production resulting from all oxidative processes. Classically, total metabolism has been divided into maintenance and production processes, with the assumption that they are more or less independent of each other, in other words, that these two parts of heat production add up. Maintenance heat production is mainly expressed as a function of body size, while the energy cost of production processes is calculated per unit of product (usually protein and fat accretion). This approach can only be accepted for an adult organism whose level of physical activity is low and who may retain some fatty tissue in some periods of life, but normally is characterized by a zero protein balance and low energy expenditure. Our general and current knowledge of bioenergetics shows that it is necessary to take into account the influence of anabolic processes on catabolism along with the impact of metabolic memory. In the field of protein metabolism the so-called anabolic increment of catabolism is a very well-known phenomenon and several research studies indicate that metabolic memory can persist for more than 30 days. From that point of view, it is unjustified to separate maintenance and productive processes, especially in growing organisms within the same taxonomic group, unless the growth rate of their body weight is taken into account. A new method of studying maintenance energy requirements in growing animals is therefore proposed. It is based on determination of the maximum body weight of an organism at which a given daily ration of food covers the maintenance requirements of a young animal but does not contain any food for growth.