β-Endorphin (βE) exerts a strong inhibitory action on plasma vasopressin (VP) of rats, after intracerebroventricular, but not after subcutaneous injection of the drug. This effect is time- and dose-dependent. Also in the water-deprived rat, this treatment leads to a strong decrease of plasma VP levels. When rats treated with histamine (HIS) intracerebroventricularly to stimulate VP levels are injected with βE to HIS treatment, βE partially prevents the increase of plasma VP levels. Naloxone subcutaneously administered, antagonizes the effect of βE in all the situations we investigated. Opioid receptors, located in the brain as well as in the pituitary, are possibly involved in these proce