The in vitro maximal stimulation by gonadotropin of cAMP accumulation in the Eel ovary is three times less in in vivo treated Fish (75 micrograms gonadotropin 18h before killing) than in controls. The existence of this phenomenon of desensitization should be taken into account for an understanding of the mechanisms of oogenesis regulation as well as for the planning of the gonadotropic treatments applied to Fish in culture.