The mammalian brain is considered to be poorly responsive to thyroid hormone after the so called "critical periods" of brain development, which occur in the rat before postnatal days 15-20. In a previous work (Muñoz, A., A. Rodriguez-Peña, A. Perez-Castillo, B. Ferreiro, J.G. Sutcliffe, and J. Bernal. 1991. Mol. Endocrinol. 5:273-280) we have identified one neuronal gene, RC3, whose expression is influenced by early neonatal hypothyroidism and thyroid hormone treatment. In the present work we show that adult-onset hypothyroidism leads to a reversible decrease of RC3 mRNA. Rats thyroidectomized on postnatal day 40 and killed three months later showed a decreased RC3 mRNA concentration in the cerebral cortex and striatum. The same effect was observed in animals made hypothyroid on postnatal day 32 and killed on postnatal day 52. RC3 expression was normal when hypothyroid animals were treated with T4 five days before being killed. In contrast, the mRNA encoding myelin proteolipid protein showed no changes in either experimental situation. RC3 mRNA levels were not affected by food restriction demonstrating that the effect of hypothyroidism was not related to the lack of weight gain. The control of RC3 mRNA is so far the only molecular event known to be regulated by thyroid hormone once the critical periods of brain development are over and could represent a molecular correlate for the age-independent, reversible alterations induced by hypothyroidism in the adult brain.