The regulation of reproduction-stage inducible melatonin biosynthesis in rice (Oryza sativa cv. Dongjin) was investigated. The flag leaf and panicle (flower) were collected from field-grown rice at three different reproductive stages: the preflowering stage, flowering stage, and postflowering stage. Melatonin synthesis was induced in the panicle, whereas no induction was observed in the flag leaf during the reproductive stages. The panicle displayed a peak melatonin level of 0.4 ng/g fresh weight (fw), which was six times that found in the flag leaf. The induction of melatonin biosynthesis was paralleled by the induction of corresponding genes and proteins such as tryptophan decarboxylase, tryptamine 5-hydroxylase, and N-acetylserotonin methyltransferase. In addition, melatonin induction was preceded by the accumulation of its precursor, tryptophan, in the panicle. These results suggest that the induction of melatonin during flower development is regulated by the transcriptional control of its biosynthesis genes and that melatonin may participate in flower development.