A new class of anti-hypertensive agent is provided by substances that inhibit the breakdown of the endogenous substances, the enkephalins and/or the endorphins. The anti-hypertensive effect of an enkephalin breakdown inhibitor is greatly enhanced by being combined with a beta adrenergic blocking agent, herein designated as beta blocker. Specifically, D-phenylalanine, en enkephalin breakdown inhibitor when used alone produces excellent blood pressure lowering in animals and man. Use of a combination of D-phenylalanine and the beta blocker propranolol provides a greatly enhanced anti-hypertensive effect in the spontaneously hypertensive rat (SHR). Blood pressure lowering by D-phenylalanine, or the latter combination, is very long lasting in the SHR and man. If the blood pressure of the rat or human is normal, D-phenylalanine has little or no effect on blood pressure. The hypotensive effect of D-phenylalanine is prevented by pretreating the SHR with naloxone or naltrexone which are specific antagonists of enkephalins or endorphins.
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