Thyroid hormone plays a key role in trophic events during development of the central nervous system. In spite of neurological and psychiatric symptoms associated with adult hypothyroidism, the role of thyroid hormone in mature brain function is less clear. In this paper we investigated the effect of thyroid status on kainic acid-induced up-regulation of mRNAs for members of the nerve growth factor family and related receptors in adult male rats by means of in situ hybridization. We found that in hypothyroid rats there is a dramatic attenuation of the kainic acid-induced up-regulation of mRNA levels for nerve growth factor, brain-derived neurotrophic factor and tyrosine kinase trkB in euthyroid rats. A trend to reduced c-fos mRNA up-regulation, which did not reach significance, was also found, whereas the increase in c-jun mRNA after kainic acid was similar in eu-, hypo- and hyperthyroid rats. These data indicate a severe impairment of the regulation of neurotrophin synthesis after excitotoxin administration in the hippocampus during adult hypothyroidism. Possible roles of thyroid hormone in molecular, biochemical and metabolic mechanisms of this defect are discussed.