Although magnetic fields can influence biological systems, including those of man and other vertebrates, no central nervous structure has been identified that might be involved in their detection. From a theoretical point of view, the pineal organ might be such a structure for the following reasons: (1) It is involved in the regulation of circadian rhythms and is thus essential for migratory restlessness ('Zugunruhe'). Orientation at that time can be altered by an artificial magnetic field (MF) with a direction differing by 90 degrees from that of the Earth. Circadian rhythms can be inhibited from phase shifting by compensation of the Earth's MF and can be influenced by an artificial MF. (2) The pineal organ is strongly dependent on its sympathetic innervation and the sympatho-adrenergic system as a whole is sensitive to magnetic stimuli. (3) The pineal organ is a light-sensitive time-keeping organ and could form part of a combined compass--solar-clock system, which has been postulated for maintaining orientation in birds. We have therefore investigated the effect of a MF on electrophysiological activity of the guinea pig pineal organ, which is a useful system for such studies on individual cells. We report here that activity was depressed by an induced MF and restored when the MF was inverted.