Disturbances of Mg2+ metabolism have been reported in association with affective disorders, seizures in eclampsia, and alcohol withdrawal. Mg2+ has been reported to have N-methyl-D-aspartate (NMDA)-antagonistic and gamma-aminobutyric acid (GABA)-agonistic properties and modulation of GABA(A)- and NMDA-dependent systems is involved in pharmacological treatment of affective disorders and seizures. We studied the effect of Mg2+ on sleep electroencephalogram (EEG) and nocturnal hormonal secretion in men. Ten normal controls were given MgSO4 (3 g MgSO4 between 2030 hours and 2100 hours, followed by 0.5 g MgSO4 per hour until 0700 hours) or placebo i.v. according to a randomized schedule. The sleep EEG was recorded from 2300 hours to 0700 hours. Blood samples were taken from 2000 hours to 0700 hours for analysis of plasma corticotropin (ACTH), cortisol, growth hormone, prolactin and melatonin. The sleep-EEG power within the spindle frequency range (11.0-12.9 Hz) showed a significant increase in the third sleep cycle, but delta power was unchanged throughout the night. ACTH concentration was suppressed between 2200 hours and 0700 hours. No changes in cortisol, growth hormone prolactin or melatonin release were found. The findings are consistent with the assumption that Mg2+ has GABA(A)-agonistic or NMDA-antagonistic effects on sleep and nocturnal hormonal secretion and hence may be useful in controlling depressive symptoms and seizures.