1. Immunoreactive beta-endorphin (IR-beta EP) 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-beta EP 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-beta EP concentration. 5. These results confirm and extend postulated dopaminergic defect in this model of hypertension.