As opposed to adults, infants were found to have 5-fold differences in values of heat production per unit of mass and unit of body surface. Animals of the same age have these differences not over 20-50%. Newborn mature infants could have these differences to reach 50-60%. The data obtained suggest significant differences in the efficiency coefficient values for muscles contractile activity to play the role of the mechanism of heat production control in chemical thermoregulation and postnatal formation of the constancy of main (standard) metabolism.