To investigate the effects of mercuric chloride ($HgCl_2$) on the differentiation of the cerebral neuron of chick embryo 9 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, cerebral proteins and adenosine triphosphate (ATP) were also analyzed. The results obtained are as follows: The ultrastructural changes in 0.5 and 1.0mg-injected groups were undetectable, but in 2.0mg-injected group, the nuclear envelops were very irregular and mitochondria, were swelled and destroyed partly. The number of polypeptide bands separated by SDS-PAGE in the normal group were 37 bands. According to the in creased dose of mercuric chloride, contends of the bands were increased in 7 bands. The activities of dehydrogenases were declined by increasing the dose of mercuric chloride. Lactate dehydrogenase (LDH) activity failed to 78% in 1.0mg-injected group and greatly to 68% in 2.0 mg-injected group. Malate dehydrogenase (MDH) activity failed to 81% in 2.0 mg-injected group. On the other hand, succinate dehydrogenase (SDH) activity decreased to 80% in 1.0 mg-injected group and greatly to 63% in 2.0 mg-injected group. ATP content in 1.0 mg-injected group was increased slightly and in 2.0 mg-injected group was increased greatly.