Background: The pineal gland has been implicated recently in the pathogenesis and clinical course of multiple sclerosis (MS). Melatonin secretion is inhibited by light and it reaches the highest levels at nights. In 85% of the MS patients, there were abnormal responses in visual evoked potential (VEP). Objective: To investigate whether there was a difference in serum melatonin level between patients with multiple sclerosis (MS) with prolonged visual evoked potential (VEP) P100 latency and patients with MS with normal VEP P100 latency. Methods: Twenty-five MS patients (16 female, 9 male) with acute attacks of MS were included in the study. The blood was collected from the patients at 03.30 am and 10.00 am for determining melatonin level. Serum melatonin level and VEP P100 latency were determined. Student's t test was used for analyzing the differences between groups. Results: Nocturnal and daytime melatonin levels and nocturnal-daytime melatonin inhibition rate were not different between 8 patients in whom VEP P100 latency was lower than 110 ms and 17 patients in whom VEP P100 latency was higher than 110 ms. Conclusion: Nocturnal-daytime melatonin inhibition rate was not different between MS patients in whom VEP P100 latency was high and MS patients in whom VEP P100 latency was normal. The present study is the first investigation on this topic.