Ten adult male rats were administered orally with thyroideum siccatum every day to produce a hyperfunctional state of thyroid hormone. They were examined electron microscopically for the fine structure of the brain. Clinically, experimental rats lost in weight, manifested a hyperactive, irritative attitude, and were in an aggressive state. Electron microscopically, the nerve cells showed a slight increase in the number of mitochondria in the frontal cortex, spiral and petaloid variation of ER in the thalamus. The specific findings in the experimental rats were variations of synapse observed in the frontal and temporal cortex, gyrus hippocampi, and thalamus, centering around the hypothalamus. They included meandering of the presynaptic membrane, formation of vacuoles by the presynaptic membrane, saccate projection of this membrane to the postsynapse, and changes in vacuoles which were torn off and fell into the dendrite. These variations occurred in the synaptic membrane and ER, probably because synthesis of the membrane had been accelerated remarkably by the action of thyroid hormones. They were considered to have brought about abnormalities to the conductive and transmission system. They seemed to be closely related with emotional disturbances and psychic symptoms, since such variations were particularly apparent in the hypothalamus.