Peptides which inhibit the secretion of gonadotropins by the pituitary gland and inhibit the release of steroids by the gonads. Administration of an effective amount prevents ovulation of female mammalian eggs and/or the release of steroids by the gonads, which can be used for therapeutic treatment. The peptides have the structure:X-R.sub.1 -R.sub.2 -R.sub.3 -Ser-Arg-R.sub.6 -R.sub.7 -Arg-Pro-R.sub.10 wherein X is hydrogen, Acr or Ac; R.sub.1 is .beta.-D-2NAL, dehydroPro or 4Cl-D-Phe; R.sub.2 is Cl-D-Phe, F-D-Phe, NO.sub.2 -D-Phe, Br-D-Phe or C.sup.a Me/Cl-D-Phe; R.sub.3 is D-Trp or .beta.-D-2NAL; R.sub.6 is D-Tyr, .beta.-D-2NAL or D-Arg; R.sub.7 is Leu or NML; and R.sub.10 is Gly-NH.sub.2, NHCH.sub.2 CH.sub.3 or D-Ala-NH.sub.2 ; provided however that R.sub.1 or R.sub.3 or both are .beta.-D-2NAL.