The pineal gland is a complex neuroendocrine organ with pronounced effects on central nervous system activity. Because previous studies in animals and humans have suggested an anticonvulsant role for the pineal hormone melatonin, we studied the day-night variations in plasma melatonin in normal children and children with febrile or epileptic convulsions. We found significant changes in day-night melatonin levels during convulsions, consistent with the hypothesis that melatonin has an inhibitory function on central nervous system activity.