Abstract
1. A method was developed for the analysis of ubiquinone in animal tissues and the recovery of added ubiquinone tested in liver of the rat, Crocodylus porosus and Squalus acanthias. 2. The ubiquinone content of heart, liver and gut (or breast muscle in birds) was measured in 67 different animal species, selected to be representative of all the vertebrate classes. 3. The suggestion is advanced that the possession of appreciable amounts of endogenous tissue ubiquinone is usually characteristic of evolutionarily advanced vertebrates, and the biological and biochemical significance of the results is discussed.