1. Immunoreactive β-endorphin (IR-βEP) was two- to three-fold higher in pituitary neuro-intermediate lobes (N-IL) of spontaneously hypertensive rats (SHR) than of normotensive Wistar—Kyoto (NT-WKY) controls.
2. Plasma levels of IR-βEP were lower in SHR than in NT-WKY rats.
3. Intravenous injections of morphine lowered blood pressure of both SHR and NT-WKY rats to the same level; naloxone restored blood pressure of both groups to pre-morphine values.
4. Infusion of bromocriptine in SHR for 1 week lowered blood pressure and N-IL IR-βEP concentration.
5. These results confirm and extend postulated dopaminergic defect in this model of hypertension.