In a study of the effect of somatostatin (growth-hormone release-inhibiting hormone) on plasma-renin in healthy volunteers, plasma-renin activity was measured by radioimmunoassay after the intravenous administration of somatostatin and also during frusemide-induced hyperreninaemia. While somatostatin was being given, basal values of renin were unchanged. Injection of frusemide alone produced hyperreninaemia; but, under somatostatin, renin release was inhibited by 45%. The results indicate that somatostatin is a potent inhibitor of renin and exerts its effect independent of sodium excretion, which was unchanged under somatostatin. Conceivably, somatostatin plays an important role in the regulation of endogenous renin release.