The aim of the present work was to assess serum melatonin levels and melatonin circadian rhythm in patients with diurnal and nocturnal complex partial epilepsy. Daily rhythms of melatonin were studied in patients with diurnal complex partial epilepsy (n=10), patients with nocturnal complex partial epilepsy (n=10), and a control group (n=10). All patients were under carbamazepine treatment. Serum melatonin samples were taken at 1000, 2200, 0100, and 0500 hours. We found that melatonin circadian rhythm was normal in all patients when compared with controls. Melatonin levels were low in both patients with nocturnal and patients with diurnal complex partial epilepsy compared with the control group at 1000, 2200, 0100, and 0500 hours; a statistically significant decrease in melatonin levels was observed in the patients with epilepsy at 1000 hours only. These findings suggest that melatonin levels and circadian rhythm of melatonin do not differ between patients with nocturnal and patients with diurnal complex partial epilepsy. Further detailed research is necessary to determine the factors that govern the nocturnal or diurnal occurrence of complex partial seizures.