As in other vertebrates, the pineal organ in salmonids receives photic information directly through specialized photoreceptor cells to synchronize both daily and seasonal behavioral and physiological events, including the reproductive. In this study, the effect of a melatonin implantation before the preovulatory period on egg quality was investigated in rainbow trout (Oncorhynchus mykiss) adults. Fish were exposed to two different lighting regimes [(constant short photoperiod (SP) of 6h light/18h dark) or natural photoperiod (NP) of 9h light/15h dark) with sub groups receiving a slow-release melatonin implant ((containing 9 mg melatonin hormone (MLT)) or sham implant. Melatonin implanted fish showed supraphysiological plasma MLT concentration during the 45 days after implantation. The spawning period started at the same time in all groups after implantation, although eggs were obtained from females exposed to SP lighting regimes only or SP lighting regimes plus implanted MLT exhibited significantly lower quality characterized by lower hatching rate and survival to first feeding than that of females expose to NP. In conclusion, it was determined that implantation of MLT in preovulatory rainbow trout females causes a decrease in egg quality. However, future investigations should be conducted to ascertain the MLT dose required for and time of implantation in rainbow trout.