GnRH peptide analogs which regulate the secretion of gonadotropins by the pituitary gland and either promote or inhibit the release of steroids by the gonads. Administration of an effective amount of a GnRH antagonist prevents ovulation of female mammalian eggs and/or the release of gonadotropins. Administration of GnRH agonists can be used to regulate fertility in male and female mammals.These and other peptide hormones exhibit improved binding efficiency and biological potency as a result having a residue in a critical, generally central location in the chain which residue contains a mixed alkyl ketone side-chain terminating in an aromatic group. Methods for efficiently synthesizing these peptides from readily available compounds are disclosed.