Melatonin ( N-acetyl-5-methoxytryptamine, aMT) is an indoleamine produced by several organs and tissues including the pineal gland. Melatonin (aMT) modulates the activity of the brain, mainly acting on both GABA and glutamate receptors. Previous studies have shown the participation of melatonin in the control of convulsive crises, suggesting that aMT concentration increases during seizures, and that patients with seizures of diverse origins show an alteration of the aMT rhythm. However, what is not known is the duration of the aMT response to seizures, and whether aMT changes during seizures could be a marker of the disease. For this reason, the serum levels of aMT in 54 children with a convulsive crisis, febrile and epileptic, were analyzed during the crisis, as well as at 1 h and 24 hours after the seizure. The results show that aMT significantly increases during the seizure (Day group, 75.64+/-45.91 and Night group, 90.69+/-51.85 pg/mL), with normal values being recovered 1 h later (Day group, 26.33+/-10.15 and Night group, 27.78+/-7.82 pg/mL) and maintained for up to 24 hours, when the circadian variation of aMT returns to the normal acrophase. Due to the interindividual variation of aMT levels among healthy people, a single determination of the indoleamine concentration is not a suitable marker of the existence of a convulsive crisis unless the circadian profile of aMT secretion in the patient is known. The results obtained also support the view that the stimulation of aMT production by the convulsive crisis may participate in the response of the organism against the seizures.