The neurophysial hormones arginine vasotocin (AVT) and isotocin (IT) are the fish counterparts of argininevasopressin and oxytocin respectively. Vasotocin precursor and isotocin precursor cDNAs were previously cloned in several fish species including chum salmon [47, 48] . In fish, AVT is involved in several physiological processes including water conservation and excretion of electrolytes [49] . However, existing data in fish correspond to the local effect, in various tissues, of circulating AVT [49] . Surprisingly, we observed that AVT precursor (avt) mRNA is expressed in the rainbow trout preovulatory ovary. To the best of our knowledge, there is no evidence of non-neural expression of avt mRNA in fish. In addition, it is noteworthy that we also observed a similar over expression of isotocin mRNA precursor in the ovary at the time of oocyte maturation. Further investigations are needed to elucidate the role of AVT and IT in the trout preovulatory ovarian functions.