Human GRF(hGRF), rat GRF(rGRF), porcine GRF(pGRF), ovine GRF(oGRF) and bovine GRF(bGRF) have been earlier characterized and synthesized. The invention provides synthetic peptides which are extremely potent in stimulating the release of pituitary GH in animals, including humans, which have resistance to enzymatic degradation in the body, and which contain the sequence: R.sub.1 -R.sub.2 -R.sub.3 -Ala-Ile-Phe-Thr-R.sub.8 -Ser-R.sub.10 -Arg-R.sub.12 -R.sub.13 -R.sub.14 -R.sub.15 -Gln-R.sub.17 -R.sub.18 -Ala-Arg-Lys-Leu-R.sub.23 -R.sub.24 -R.sub.25 -Ile-R.sub.27 -R.sub.28 -R.sub.29 -R.sub.30 -R.sub.31 -R.sub.32, wherein R.sub.1 is Tyr, D-Tyr, Met, Phe, D-Phe, pCl-Phe, Leu, His or D-His having either a C.sup.a Me or N.sup.a Me substitution or being unsubstituted; R.sub.2 is Ala, D-Ala or D-NMA; R.sub.3 is Asp or D-Asp; R.sub.8 is Ser, Asn, D-Ser or D-Asn; R.sub.10 is Tyr or D-Tyr; R.sub.12 is Arg or Lys; R.sub.13 is Ile or Val; R.sub.14 is Leu or D-Leu, R.sub.15 is Gly or D-Ala; R.sub.17 is Leu or D-Leu; R.sub.18 is Tyr or Ser; R.sub.23 is Leu or D-Leu; R.sub.24 is His or Gln; R.sub.25 is Glu, Asp, D-Glu or D-Asp; R.sub.27 is Met, D-Met, Ala, Nle, Ile, Leu, Nva or Val; R.sub.28 is Asn, Ser or desR.sub.28 ; R.sub.29 is Arg, D-Arg or desR.sub.29 ; R.sub.30 is Gln or desR.sub.30 ; R.sub.31 is Glu or desR.sub.31 ; and R.sub.32 is Gly or desR.sub.32. These peptides as well as their nontoxic salts may also be used diagnostically.