Daily dosage of 100 mg/kg of diphenylhydantoin was administered on the ep-mouse for 3 weeks and cholinesterase activity, water, Na, K, glutamic acid, GABA (γ-aminobutyric acid) and glutamic decarboxylase activity in cerebrum were followed during the course. 1. The ChE activity of the ep-mouse at rest appeared to be lower than that of the normal mouse. Diphenylhydantoin manifested an increment of the ChE activity at the end of the first week and, in contrast, a decrement at the end of of the 3rd week of its administration course in both groups. It is, however, interesting to note that the variation of both directions was found to be greater in the ep-mouse than in the normal mouse. 2. The content of total water in the ep-mouse was more than that in the normal mouse. Diphenylhydantoin did not make any change in the water content in either groups. 3. The total Na and K contents were greater in the ep-mouse than in the normal mouse. The administration of diphenylhydantoin did not affect their contents in the normal mouse, but showed a decrement in both contents down to the normal level in the ep mouse. 4. The cerebrum of the ep-mouse contained more glutamic acid and, in contrast, less GABA than that of the normal mouse. After the diphenylhydantoin administration, glutamic acid was found to be decreased and GABA to be increased in both mice. It is to be noted that the difference in the contents of gluamic acid and GABA between the ep-mouse and the normal mouse was approaching each other in the course of the administration. 5. Glutamic decarboxylase activity was lower in the ep-mouse than in the normal mouse. In the course of diphenylhydantoin administration, the activity was found to be increased beyond the level of the normal mouse at the end of the first week and to be decreased down to the normal level at the end of the 3rd week in both mice.