To investigate the effects of mercuric chloride ($HgCl_2$) on the differentiation in the cerebral neuron of chick embryo 7 days, the ultrastructural changes in nerve cells injected with a various doses of mercuric chloride were observed with transmission electron microscope. The enzyme activity of the some dehydrogenases, and adenosine triphophate (ATP) were also analyzed. The results obtained are as follows; The ultrastructural changes in 1.0mg-injected group, the nuclear envelope were irregular, and the RER, Golgi complexes and mitochondria were not well developed. In 2.0mg-injected group, the nuclear envelope were partly destroyed or detached, and mitochondria were decreased in number and their cristae were destroyed, too. The RER and Golgi complexes were less developed than those of the normal groups. In general, the activities of dehydrogenases were declined by increasing the dose of mercuric chloride. Lactate dehydrogenase (LDH) activity fatted to below 85% of the normal group in 1.0mg-injected group, and 69% in 2.0mg-injected group. Malate dehydrogenase (MDH) activity was decreased greatly to 76% in 2.0mg-injected group. Succinate dehydrogenase (SDH) activity fatted to 85% in 1.0mg-injected group, and 74% in 2.0mg-injected group. ATP content in 1.0mg-injected group was almost near to the normal level, but it was increased significantly in 2.0mg-injected group.