The LH and FSH activities of equine choriogonadotropin (eCG) have been compared in several species with those of the highly purified homologous pituitary gonadotropins. The molar FSH/LH activity ratio of eCG determined by RRA is 0.20 in the pig, 0.25 in the rat and 0 in the horse. These data demonstrate the LH monospecificity of eCG in its own species as it is the case for hCG. We have also shown that equine LH exhibited a FSH-activity similar to that of eCG in the pig and in the rat but not in the horse. In the female rat, the binding activity to FSH receptors and the in vitro FSH activity of eCG are similar (about 0.005 X eFSH). In the male rat, the binding activity of eCG to FSH receptors is much higher (0.15 X eFSH) than its in vitro FSH activity (0.007 X eFSH). The behaviour of equine LH is similar to that of eCG in the four systems. Our data clearly indicate that the double activity of eCG is not unique to this molecule. Moreover, the double activity is not intrinsic to the eCG molecule since it depends on the species and the sex of the animal used as well as on the system used (RRA) or in vitro biological assay).