Chicken II and salmon GnRH analog decapeptides resistant to degradation by peptidase incorporating D-arginine, D-leucine, D-tBu-Serine, D-Trp or other active D amino acids at position 6 and ethylamide, aza-Gly-amide or other Gly amide at position 10. The analogs demonstrate preferential binding to male and female reproductive system GnRH receptors. Biopotency is greater within the reproductive system than at the pituitary. These non-mammalian GnRH analogs may be used in pharmaceutical preparations, and specifically in various treatment methods as a contraceptive or post-coital contraceptive agent. The non-mammalian GnRH analogs are also provided in pharmaceutical preparations that may be used clinically for maintaining pregnancy when used in very low doses and administered in pulsatile fashion, as well as in preparations for the treatment of male and female reproductive system disorders. The aza-Gly (10) amide non-mammalian analogs are yet other embodiments of the non-mammalian GnRH analogs provided as a part of the invention.