In deer, most of the earlier investigations on pineal function examined the effects of artificial photoperiods or the administration of melatonin to manipulate reproduction. However, endogenous melatonin rhythms have not been studied in red deer. Thus, we monitored seasonal changes in plasma melatonin concentrations in 16 adult female Iberian red deer living in outdoor enclosures. Blood was sampled on the day of each seasonal change every 3-4 hr overnight and 1 hr before and after sunset and sunrise. In addition, in six of the previous hinds, blood sampling during the hour prior and after sunset and sunrise was collected every 20 min. Significant differences were found both in amplitude and duration of the nocturnal plasma melatonin profiles in the four seasonal changes (P < 0.01). The nocturnal mean level of melatonin, the duration of nocturnal secretion levels and maximal concentrations were significantly higher at the winter solstice than in summer solstice or equinoxes (P < 0.05). Moreover, the mean overnight concentrations were significantly higher at the spring equinox and winter solstice than during the summer solstice and autumn equinox (P < 0.05). A pronounced elevation from low levels was recorded 1 hr after sunset, remained elevated during the hours of darkness and declined to low levels 1 hr after dawn. Concentrations close to sunrise were higher than those near sunset at all changes of season (P < 0.05). These results show for the first time in red deer that the pineal gland of the adult female is highly responsive to both daily and seasonal changes in natural environmental illumination, although overnight levels lasted longer than the photoperiodic night is all cases, particularly at the winter solstice.