The quantitation of thyroid hormone levels in cerebrocortical synaptosomes of the adult rat at altered thyroid states was studied because of the recent idea of the involvement of thyroid hormone in the adult mammalian brain. In contrast to a marked fall in serum T3 levels in PTU-induced hypothyroid rats, the synaptosomal T3 content was raised by 9.5 fold in this hypothyroid situation compared to the euthyroid control rats. A single injection of T3 (2 micrograms/g) to the hypothyroid rats lowered the T3 level to about half of those values obtained in only hypothyroid condition. The T4 levels remained below the detectable range in all the groups tested. This may be because 70% to 80% of the intraneuronal T3 comes through the local deiodination of T4. The present investigation indicates a tendency for synaptosomal reorientation of T3 level for vital neurophysiologic function in altered thyroid conditions.