The purpose of this study was to investigate the differentiation and degeneration of neurons in developing chick embryo. The activity of acid phosphatase(ACP) was measured and cytochemical study of ACP and ultrastructural changes were observed in prosencephalon, mesencephalon and rhombencephalon from day 4 to day 19 of incubation. As a result, the activity of ACP of all brain region was tend to increase from day 4 to day 19. On day 13, activities of ACP of mesencephalon and rhombencephalon were increased greatly and activity of ACP was decreased each region on day 17. On electron microscopic examination, the reaction product of ACP were localized at GERL complex, lysosome, Golgi body and vacuoles of neurons. Morphologically, disrupted nuclear envelope, mitochondrial destruction, vacuolization and ribosomal crystalization were observed.