Grass puffer (Takifugu alboplumbeus) exhibits unique spawn ing migration: it spawns on beach in semilunar cycles during spring tide in early summer. The fish aggregate at certain seashore locations several hours before high tide every two weeks. Observations of spawning behavior in spawning grounds and in aquarium revealed that the spawning rhythm is tightly connected to the tidal changes and is also endoge nously maintained possibly under the control of lunar- or tid al-related clock. In the hypothalamus, the genes for gonadotro pin-releasing hormone, kisspeptin (kiss2) and gonadotropin-inhibitory hormone (gnih) show specific daily and circadian oscillations in expression. The treatment of me latonin significantly stimulates the expression of these genes, suggesting that melatonin is involved in the daily variations of these genes. In addition, the expression of cryptochrome gene, one of the components of the circadian clock machinery, shows a lunar-synchronized change during the spawning pe riod. Interestingly, kiss2 and gnih showed specific changes in expression in a lunar cycle during the spawning period. More over, in the pineal gland, the expression of melatonin receptor genes showed ultradian variations with a period of 15 hours under constant dark conditions, suggesting that there are two different biological clocks such as circadian and circatidal clocks. These clocks and melatonin may play important roles in transmitting the periodic information of tidal and moonlight changes to the reproductive neuroendocrine center in the hy pothalamus of the grass puffer.