ABSTRACT The origin of the large reserves of vitamin A which may be found in certain tissues, particularly the livers, of fish has been the subject of considerable study. In the case of some species (e.g. cod and halibut) an examination of the food has shown that the concentrations of vitamin A and its acknowledged precursor carotene are curiously small (Drummond and Hilditch, 1930; Lovern and Sharp, 1933). This striking fact has even led to the suggestion that some fish possess the power to synthesise vitamin A from unknown precursors. Alternatively, it seems not improbable that although the food may itself contain but slight traces of vitamin A, the retention of the latter by the liver might in time lead to an accumulation sufficient to account for the large reserves which are encountered.