In situ hybridization histochemistry was used to locate cells containing tyrosine hydroxylase (TH) mRNA in the hypothalami of salt-loaded and Brattleboro rats. The hyperosmotic plasma conditions found in these animals, as compared to control animals, was associated with an increase in detectable TH mRNA-producing cells in the paraventricular and supraoptic nuclei. These results suggest that dopamine synthesized by neurons of those nuclei may participate in the regulation of neuropeptide synthesis and release within the nuclei and the posterior pituitary.