Polypeptides of the formula: ##STR1## in which X.sub.1 is Phe, D-Phe or C.sub.6 H.sub.5 CH.sub.2 CH.sub.2 CO--;X.sub.2 is Phe, Tyr, Trp, Met or Leu;X.sub.3 is Trp or D-Trp;X.sub.4 is Thr, Val, .alpha.-Abu or Phe;andX.sub.5 is Phe, D-Phe or --NHCH.sub.2 CH.sub.2 C.sub.6 H.sub.5 ;the linear precursor intermediates thereof and pharmaceutically acceptable salts and amides thereof are selective inhibitors of growth hormone release without materially altering blood serum levels of glucagon or insulin. In addition, the above-described compounds are active growth hormone suppressants for periods as long as two hours.