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Seven active tetrapeptide amides characterized by a C-terminal phenylalanyl aminoadamantane (PheNHAd) sequence, were identified by selective testing for human renin inhibitory activity among compounds with adjacent hydrophobic groups and molecular size equivalent to 3-5 amino acid residues. The new inhibitors were compared with known renin inhibitors (RIP, pepstatin, H-77) and opioid analgesic agents (Met-enkephalin, morphine), with the following results: The new inhibitors were active against human renin (IC50 approximately 10-5M), but inactive against rat renin and pepsin. Although active in opiate receptor binding studies (IC50 approximately 10(-7)M), they were, with few exceptions, inactive in the mouse writhing and hot plate tests for analgesia. SAR studies suggested a separation of the renin inhibitory from the analgesic activity of enkephalin analogs. Preliminary experiments with sodium-depleted rhesus monkeys indicated hypotensive activity for three of the new inhibitors at 3 mg/kg i.v., and RIP at 1 mg/kg. The recently reported clinical hypotensive properties of RIP (Zusman et al., Trans. Assoc. Am. Physicians 96:365, 1983) along with the present comparative studies suggest that the new inhibitors may lead to clinically useful agents.