The patterns of two specialized and functionally relatedenzymes were investigated in the developing frog (Rana temporariaand Rana catesbeiana). Xanthine dehydrogenase (E.C. 1.2.1.) anduricase (E. C. 1. 7 . 3 . 2.) were found to remain apparently constantduring the first ten days of normal development, both rising rapidlythereafter, beginning at the same larval stage. Comparison withenzymes of similar developmental pattern suggests this phase-specificincrease is in preparation for the onset of digestive activity.
The same parallelism between uricase and xanthine dehydrogenasewas observed when determinations were extended to the organlevel. Each enzyme was found in quantity in liver and kidney, and intrace amounts, all of about the same magnitude, in several otherorgans tested.
By detecting radioactive tracer amounts of product, verylow levels of xanthine dehydrogenase were measured in embryonicstages, probably corresponding to the order of small numbers ofmolecules per cell.
Evidence was gathered to suggest that embryonic uricaseaccumulation is neither induced by its substrate nor involved withthe subcellular particles in which the enzyme is found.
