Gonadotrophin-releasing hormone (GnRH) stimulates the pituitary secretion of both luteinizing and follicle-stimulating hormones, and thus controls the hormonal and reproductive functions of the gonads. GnRH analogs, which include agonists and antagonists, have been produced by amino acid substitutions within the native GnRH molecule resulting in greater potency and a longer duration of effectiveness. While the initial antagonists produced significant side effects, more recent potent, long-acting, water-soluble, low histamine-release third-generation compounds such as cetrorelix, abarelix, azaline B and acyline have appeared. Differently to GnRH agonists, antagonists competitively block and inhibit GnRH-induced GnRH receptor gene expression leading to an immediate, dose-dependent, pituitary suppression without an initial stimulation of the gonadal axis. The aims of this review are to compare the effects of GnRH agonists vs antagonists and to describe the existing literature concerning new antagonists in domestic carnivores. In male dogs, a single subcutaneous dose of acyline safely and reversibly decreased serum gonadotrophins and testosterone concentrations for 9 days and prevented physiological response of gonadal the axis to agonistic challenge for 14 days. The same protocol reversibly impaired spermiogenesis, spermatocytogenesis and semen quality in both cats and dogs. In females, third-generation GnRH antagonists prevented ovulation and interrupted pregnancy in canids but not in felids. During anestrus, a single acyline injection exhibited limited prevention of the ‘flare-up’ effect in GnRH agonist-implanted bitches. Although GnRH antagonists appear to have a promising future in domestic carnivores reproduction, the information is still scarce and further work is needed before they can be widely recommended.