The pineal gland is known to play a central role in the photoperiodic control of reproduction in seasonal breeders. The present review, based on experimental data obtained in four different species of hamster, the Syrian or golden, the Djungarian or Siberian, the Turkish and the European, attempts to evaluate the role of the pineal in this phenomenon, the message which is conveyed from this gland as well as the mechanism of action of this message. Melatonin one of the 5-methoxyindoles rhythmically synthesized in the pineal gland, appears to be the pineal hormone conveying the photoperiodic message. The importance of the duration of the nocturnal peak of circulating melatonin is now well established, but our knowledge on the sites of action and on the mechanisms of action of melatonin is still rather poor. The presently available data suggest that melatonin can act at different levels, either on specific receptors, on receptors of other transmitters, on various molecular processes after diffusion in the cell or indirectly via effects on other endocrine hormones, e.g. gonadal steroids.