Chronic osmotic stimulation influences the hypothalamo-adenohypophysial axis by inhibiting the synthesis of hypothalamic corticotrophin-releasing factor (CRF-41) and subsequently the secretion of basal and adrenalectomy-elevated adrenocorticotrophin from the adenohypophysis. In the present study, we used a substance P antagonist to test the hypothesis that this inhibition is mediated centrally by substance P or other tachykinins. In control rats and rats given 2% saline to drink for 12 days, intracerebroventricular administration of a substance P antagonist elevated plasma adrenocorticotrophin and corticosterone levels. Using quantitative in situ hybridization histochemistry, it was also demonstrated that CRF mRNA increased in the medial parvocellular division of the paraventricular nucleus of saline-treated as well as control rats 6 h after intracerebroventricular administration of the antagonist, while vasopressin mRNA in the medial parvocellular division of the paraventricular nucleus was increased in the control animals only. These results provide evidence that central endogenous substance P has an inhibitory influence over the synthesis and release of CRF-41 both under normal conditions and during a chronic osmotic stimulus.