The qO2 or oxygen uptake of a tissue is considered to be a reflexion of the metabolic energy produced through substrate catabolism to support its functional activities. It is usually accepted that the total in vivo oxygen consumption is directly related to ATP production as the sole energy transfer and storage mechanism for endergonic processes1. Energy balance calculations in biology make use of this assumption, which is based on the known in vitro efficiencies of these processes. Recently biochemists have shown that certain non-mitochondrial enzyme systems assimilate or “take up” oxygen2 and that respiration without phosphorylation is probably essential to many of the biological functions of mitochondria3. The relative contribution of non-phosphorylating and non-mitochondrial oxygen uptake to total respiration in vivo has been studied in the isolated perfused rat heart. Oligomycin and cyanide were used as inhibitors to distinguish mitochondrial but non-phosphorylating respiration from non-mitochondrial respiration.