Bull. Jpn. Soc. Fish. Oceanogr. 74(sp), Page 66-83, 2010   Endocrine regulation of reproductive cycles in fish Michiya Matsuyama Department of Bioresource Sciences, Faculty of Agriculture, Kyushu University, 6-10-1 Hakozaki, Fukuoka, Fukuoka 812-8581, Japan e-mail: rinya_m@agr.kyushu-u.ac.jp This paper aims at reviewing recent information regarding the endocrine regulation of sexual maturation and reproductive cycles in teleost fish. Reproduction in fish, as well as in other vertebrates, is controlled by the endocrine system of brain-pituitary-gonad (BPG) axis. Melatonin contributes to synchronizing behaviors and neuroendocrine regulations with the annual and daily variations in photoperiod. Though many data on melatonin effects on crucial neuroendocrine regulator in fish reproduction have been shown, little is known regarding the mechanism of melatonin action on the BPG-axis. It is clear that the major regulators during sexual maturation are the GnRH, pituitary gonadotropins (LH and FSH) and sex steroids. Recent studies, however, show more complex endocrine regulation; numerous circulating endocrine and locally-acting neuroendocrine factors regulate the various stages of sexual maturation and spawning cycle. Moreover, some actors have entered the field of reproductive physiology. For example, KiSS system, found in mammals, seems to be central to the regulation of GnRH, and consequently LH and FSH secretion in fish. The basic mechanism underlying sexual maturation in fish would be progressed by using model species such as medaka and zebrafish; however, in fact, the controlling system of BPG-axis depends on species specificity. From the viewpoint of fishery management, it is needed to clarify the reproductive characteristics including maternal effects on the survival and recruitment of offspring in each species. In the final chapter, I am emphasizing the necessity to develop the captive experimental system of small pelagic fish to understand the change of reproductive characteristics controlled through the BPG-axis. Key words: reproductive cycle, endocrine regulation, kisspeptin, GnRH, GTH, steroid hormones