The consequences of immunization against specific regions of gonadotropin receptors were analysed in rainbow trout, on gonadal development and plasma testosterone levels. Males immunized against LHR alone or in combination with FSHR, showed a decrease in mean GSI. related to an inhibition of the spermatogenetic process; females immunized against FSHR or LHR exhibited delayed vitellogenesis. Vaccination against FSHR, and to a less extent against LHR, had a global inhibitory effect on plasma testosterone levels.