1. Tissues of a number of avian and mammalian species have been examined for the presence of 2-polyprenylphenols7 6-methoxy-2-polyprenylphenols, 6-methoxy-2-polyprenyl-1,4-benzoquinones and 5-demethoxyubiquinones. I n no instance was evidence found of the presence of any of these compounds, although most tissues contained a t least two ubiquinones and one ubiquinol.2. Tracer studies with g~-hydroxy[U-~~C]benzoic acid have been carried out to detect the precursor of ubiquinone-9, "5-demethoxyubiquinone-9", reported by Olson and co-workers [14,15] in rat liver. The results obtained provided good evidence that this compound is ubiquinol-9.As in the cases of gram-negative bacteria, green algae, higher plants, moulds, protozoa and yeasts [l--81, p-hydroxybenzoic acid has been shown to be a most effective precursor of ubiquinones in higher animals (monkey, mouse and rat) [3,4,9,10]. The intermediates involved in the conversion of this acid to ubiquinones by higher animals have still to be defined, although there would appear to be some evidence that they are the same as those which participate in the biosynthesis of ubiquinones by photosynthetic and some non-photosynthetic gramnegative bacteria (Scheme 1) [ll-131. Thus, Olson COzHCOz H I Iand co-workers have isolated a precursor of ubiquinone-9 from rat liver which they have suggested is 5-demethoxyubiquinone-9 (V, n = 9) [14,15]. I n addition, these workers have presented some evidence for the presence in rat liver of 6-methoxy-2-nonaprenylphenol (111, n = 9)

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