Temperature regulation of chemotraphs (animals and humans) is one of the most sophisticated processes in nature. The basal condition is characterized as the absolute minimum energy state during consciousness. The conditions necessary are: no food for twelve hours, restful sleep, complete rest when awake, no excitement, and an ambient temperature of 20–27°C. The body’s metabolism of chemotraphs is constantly producing excess heat, even at rest that must be removed in order to maintain thermal equilibrium. Under normal conditions the heat produced by the bio-chemical reactions within the body must be the same as the heat loss. A first law analysis is conducted in determining the net rate of heat-loss while the body is at rest. The results are then compared with data obtained by physiologists for humans and checks to within 5%. The analysis is extended to predicting the occurrence of fever. Dimensional results are then presented for the body temperature vs time. Fever can occur either through increased slow oxidation/combustion (metabolic oxidation) of a mix of carbohydrates and fats, or by reduced heat-loss from the skin due to reduced blood flow (which causes shivers). If all the energy released during oxidation/combustion results in direct heat production then it is possible to predict I) temperature vs time, and ii) the initial metabolic rate from nasal exhaust measurements during initial periods of fever. Good agreement has been obtained between model results and experimental data on cats during heat-up periods.