An increased cancer incidence has been reported in individuals living and/or working in an environment in which they are exposed to higher than normal artificial electromagnetic fields. One of the most uniform changes associated with the exposure of animals to either pulsed static geomagnetic fields or to sinusoidal extremely low frequency magnetic fields has been a reduction in high night-time levels of melatonin. Melatonin is a hormone produced especially at night in the pineal gland, a pea-sized organ near the center of the human brain. The high nocturnal production of melatonin leads to elevated blood melatonin levels at night as well. The exposure of humans or animals to light (visible electromagnetic radiation) at night rapidly depresses pineal melatonin production and blood melatonin levels. Likewise, the exposure of animals to various pulsed static and extremely low frequency magnetic fields also reduces melatonin levels. Melatonin is a potent oncostatic agent and it prevents both the initiation and promotion of cancer. Reduction of melatonin, at night, by any means, increases cells' vulnerability to alteration by carcinogenic agents. Thus, if in fact artificial electromagnetic field exposure increases the incidence of cancer in humans, a plausible mechanism could involve a reduction in melatonin which is the consequence of such exposures.