The rapid growth of brain during ontogeny affords a valuable experimental tool for studying the factors affecting protein breakdown and turnover. Despite the wealth of information documenting the remarkable changes during developmental(1–5) the correlation between chemical events and functional changes is poorly understood. Incorporation experiments with labeled amino acid precursors have established that brain proteins are in a state of dynamic equilibrium. The aspects concerned with the synthetic mechanisms are covered by Murthy(6) and by Roberts et al.(7) The purpose of this report is to comment on the areas concerned with protein breakdown, which is a necessary sequel to synthesis to maintain turnover. Other studies concerned with development are an earlier report from this laboratory,(3) the report of Oja,(8) and the contribution of Palladin and Belik.(9)