﻿AbstractThis study contains an analysis of basal metabolic rate (BMR) in 1817 endothermic species. The aim was to establish how metabolic scaling varies between the main groups of endotherms during evolution. The data for all the considered groups were combined and the common exponent in the allometric relationship between theBMRand body weight was established asb= 0.7248. Reduced to the common slope, the relative metabolic rate forms the following series:Neognathae–Passeriformes– 1.00,Neognathae– Non-Passeriformes– 0.75,Palaeognathae– 0.53,Eutheria– 0.57,Marsupialia– 0.44, andMonotremata– 0.26. The main finding is that the metabolic rate in the six main groups of mammals and birds consistently increases as the geological time of the group’s divergence approaches the present. In parallel, the average body temperature in the group rises, the duration of sleep decreases and the duration of activity increases.BMRin a taxon correlates with its evolutionary age: the later a clade diverged, the higher is its metabolic rate and the longer is its activity period; group exponents decrease as group divergence nears present times while with increase metabolic rate during activity, they not only do not decrease but can increase. Sleep duration in mammals was on average 40% longer than in birds whileBMR, in contrast, was 40% higher in birds. The evolution of metabolic scaling, body temperature, sleep duration, and activity during the development of endothermic life forms is demonstrated, allowing for a better understanding of the underlying principles of endothermy formation.