The considerable amount of information about enzyme concentrations in chick organs which accumulated in the literature has, up to now, not been adequately put together and interpreted. This was the purpose of the present review. We classified enzymes in four chick organs (liver, brain, heart and kidney) on the basis of the developmental stage at which they emerge, scrutinized the physiological relevance of the changing enzyme concentrations and compared this material with analogous information about developing mammalian organs. It is hoped that the system designed here for an objective assessment of the process of enzymic differentiation will benefit future studies of biochemical ontogeny in higher animals.