In Porcellio dilatatus, ovarian protein synthesis shows a diurnal rhythm with a maximum at the onset of night. Sham-operated females maintain this rhythm, which disappears after the ablation of the central part of the protocerebron. This operation also induces a decrease in the mean level of ovarian protein synthesis. Protocerebron implantation, before the removal of the brain, does not prevent the loss of that rhythm but maintains the same synthesis level as in sham-operated animals. Control of the ovarian protein synthesis rhythm by the central nervous system and the existence of a stimulating neurohormone for that synthesis are discussed.