Melatonin, released from the pineal gland, is an important signal within the circadian pacemaking system of passeriform birds. Until now, seasonal variations in melatonin production have only been examined in a few avian species and the role of melatonin in the regulation of annual rhythms in birds is unclear. We investigated plasma melatonin in a group of house sparrows kept in an outside aviary in spring (March/April), summer (May/June), autumn (September/October), and winter (December/January). The durations of elevated melatonin values mirrored the seasonal changes in night length to a certain degree, the melatonin signal being longest in winter and shortest in summer. Additionally, plasma melatonin peak amplitudes differed significantly among seasons, with highest values in spring and summer and lowest values in winter. Cultured explanted pineal glands obtained from animals in winter and summer showed patterns of in vitro melatonin release comparable to in vivo circulating melatonin with different durations of elevated melatonin and peak amplitude values. These data indicate that the circadian pacemaking system of the house sparrow changes properties seasonally, either as a result of endogenous mechanisms or in response to environmental conditions. These properties are maintained in the pineal gland even after isolation from the animal.