The effects of light or artificial earth-strength magnetic fields (MFs) on pineal melatonin synthesis were studied in the Pied Flycatcher, a migratory bird. Serum melatonin levels exhibited a circadian rhythm with the highest values during the night and lowest during the day. Nocturnal melatonin synthesis was significantly diminished following exposure to light during the night. It was reduced by a similar extend after exposure to an artificial MF that simultaneously reversed the horizontal component of the field and altered its intensity. In contrast, melatonin levels were not significantly changed when the bird was exposed to a MF that changed only in intensity or direction.