Recovery of nocturnal melatonin concentration takes place within one week following cessation of 50 Hz circularly polarized magnetic field exposure for six weeks.

In our earlier paper [Kato et al. (1993): Bioelectromagnetics 14:97-106], melatonin concentration in pineal gland and blood was reported to be suppressed after exposure to a circularly polarized 50 Hz magnetic field for 6 weeks. In the present series of experiments, we investigated the time course of recovery after cessation of the exposure. Rats were exposed to a circularly polarized 50 Hz magnetic field at 1 microT for 6 weeks, and the melatonin concentration of blood of separate groups was determined at the end of the exposure and at 1 week and 4 weeks following cessation of exposure. Nocturnal melatonin concentration was reduced (P < 0.05) after 6 weeks of exposure. Melatonin concentration at 1 week following cessation of exposure was normal, and no further change was observed 4 weeks later.