AbstractAction of prolactin in intact and hypophysectomized eels. IX Effect on N‐acetyl‐neuraminic acid (ANAN) content of the skin, in sea waterIntact male silver eels were gradually transferred from fresh water to sea water. The ANAN content of the skin is significantly reduced after 50 days in sea water. Ovine prolactin injected for 10 days is without effect on skin mucus.Eels were hypophysectomized the day before their transfer; after 20, 30 and 50 days in salt water, ANAN content is progressively reduced; prolactin injection starting just after hypophysectomy has no effect after 20 days, prevents the ANAN decrease after 30 days and significantly increases skin ANAN after 50 days. These effects are compared with those obtained after similar treatments in fresh water: in both media, mucus production is stimulated after prolactin treatment in hypophysectomized eels, but is not increased in intact animals. Simultaneous plasma sodium changes are discussed in the light of recent data on eel osmoregulation.RésuméChez l'Anguille intacte transférée graduellement en eau de mer, la teneur en ANAN de la peau subit une chute lente, significative au 50e jour. l'injection de prolactine pendant 10 jours n'a pas d'effet net.l'hypophysectomie pratiquée la veille du transfert en eau de mer provoque une diminution significative de la concentration en ANAN, 20, 30 et 50 j après l'opération. La prolactine ovine, injectée aussitôt après l'ablation, reste sans effet apparent après 20 j, puis normalise (30 j) et accroît nettement (50 j) le taux de l'ANAN cutané, reflet de la quantité de mucus. Ces réponses sont comparées à celles obtenues en eau douce, ainsi qu'aux variations de la natrémie dans ces deux milieux et discutées à la lumière de données récentes sur l'os