We investigated the effects of thyroid status on nitric oxide synthase (NOS) gene expression in the rat hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). Propylthiouracil (PTU)-induced hypothyroidism in male rats produced a highly significant reduction in NOS gene transcripts in the PVN and SON, as assessed by quantitative in situ hybridization histochemistry with a specific oligodeoxynucleotide probe. The addition of T3 (40 micrograms/kg) to the PTU-containing diet completely prevented the reduction in NOS transcripts. Hyperthyroidism, induced by adding 160 micrograms/kg T3 to the food, more than doubled the prevalence of NOS transcripts in the PVN and SON after a similar time. Up-regulation of NOS gene transcripts induced by the osmotic stimulus of chronic salt loading was markedly attenuated by PTU-induced hypothyroidism. These results demonstrate a major effect of thyroid status on regulation of NOS gene expression in the hypothalamus.