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The reindeer (Rangifer tarandus tarandus) is a semidomesticated holarctic ruminant subject to economic activity. Our research was carried out in Northern Finland, at latitude 69 [degrees] 10 [minutes] N, where the daily light-dark rhythm has a polar pattern: constant darkness starts on November 26th and lasts about 10 weeks, while the polar day of about 10 weeks starts on May 15th. We had previously found distinct daily rhythms in reindeer melatonin secretion in autumn, winter and spring but not at all in summer. Here we determine how the daily melatonin pattern develops after the period of polar day and how it disappears at the beginning of that period. Melatonin showed increased concentrations very soon after the first sunset. Thereafter the daily pattern developed gradually and was fully developed around the autumn equinox. Its shape was asymmetric and the maximum amplitude occurred near the end of the dark period. The disappearance of this daily pattern in spring was also gradual. The maximal concentrations depended on ambient illumination, especially the duration of the dark period. In spring the reindeer appeared to be more sensitive to light than in autumn. Additionally illumination of the same magnitude suppressed serum melatonin levels more effectively before midnight than afterwards, which is in accordance with the asymmetric secretion pattern. Melatonin secretion was always suppressed when the ambient illumination exceeded 1,000 lx. The pattern of melatonin secretion in reindeer is different from those described earlier and hence the reindeer may serve as a model for melatonin secretion at high latitudes. We suggest that, in addition to the duration of the melatonin secretion, changes in amplitude are also of importance in the reindeer.