Reports from recent epidemiological studies have suggested a possible association between extremely low frequency (ELF; including 50- or 60-Hz) electric- and magnetic-field exposure, and increased risk of certain cancers, depression, and miscarriage. ELF field-induced pineal gland dysfunction is a possible etiological factor in these effects. Work in our laboratory and elsewhere has shown that ELF electromagnetic-field exposure can alter the normal circadian rhythm of melatonin synthesis and release in the pineal gland. Consequences of reduced or inappropriately timed melatonin release on the endocrine, neuronal, and immune systems are discussed. Laboratory data linking ELF field exposure to changes in pineal circadian rhythms in both animals and humans are reviewed. The authors suggest that the pineal gland, in addition to being a convenient locus for measuring dyschronogenic effects of ELF field exposure, may play a central role in biological response to these fields via alterations in the melatonin signal.