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.

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