Data from the author's laboratory on relationships between structure and function of equine luteinizing hormone, follicle-stimulating hormone and choriogonadotrophin as well as their mechanisms of action are reviewed and compared with their human counterparts. Polymorphism of these hormones and problems associated with their purification are discussed as well as the association and dissociation of their alpha- and beta-subunits. The affinity of receptor binding, the superactivity of membrane transduction and homologous desensitization of target cells by non-stimulatory doses of the gonadotrophins are also reviewed.