Weak magnetic fields change extinction of a conditioned reaction and daytime melatonin levels in the rat.

Altogether 100 albino rats were conditioned to flee after an acoustic stimulus (1000 Hz). During the following retention period (extinction of the reaction), half of the animals (test group) were exposed to a vertically directed, weak and extremely low-frequency magnetic field (10 Hz, 10(-4) T). Under the field's influence, the animals showed an increased retention of conditioned reactions and had an elevated level of serum melatonin compared with the animals of the control group. These effects disappeared when the investigation was performed in the dark.