Circadian and seasonal rhythms are a fundamental feature of all living organisms. The functional mechanism involved is built around internal biological clock(s) and the hormone melatonin (Mel) is one of its critical components. Although numerous other sources have been identified (retina, harderian gland, gut), in vertebrates Mel is primarily produced by the pineal gland during the dark period of the light-dark cycle. This rhythmic Mel is generated directly by circadian clock(s). The Mel rhythm is thus an important efferent hormonal signal from the clock. The periodic secretion of Mel might thus be used as a circadian mediator of a system that can 'read' the message. The duration of the nocturnal Mel production is directly proportional to the length of the dark period. It is through these changes in duration that the brain integrates the photoperiodic information. In essence, the Mel rhythm appears to be an endocrine code of the environmental light-dark cycle conveying photic information that is used by organisms for both circadian and seasonal temporal organization. The major question arising from this effect of Mel concerns it precise mechanism of action. From the data reported in the present minireview, it appears that the photoneuroendocrine mechanism is not fundamentally different in vertebrates at least as far as the role of Mel is concerned.