Objective: Plasma serotonin is the fraction of total serotonin released mainly by enterochromaffin cells in the gastrointestinal tract. The circadian rhythm of peripheral serotonin is less studied than that of serotonin in the brain. We studied the 24-h profiles of circulating serotonin concentrations in relation to meal ingestion in normal adults. Methods: Blood samples were drawn 39 times throughout a 24-h period from 10 healthy male subjects provided with meals on a fixed schedule. Five subjects were lean and the other five were obese (BMI > 25) according to Asia-Pcific obesity criteria by WHO. On a separate day, four subjects (two lean, two obese) were fasted and then provided with an unexpected meal to clarify the effect of meal consumption on plasma serotonin levels. Results: Plasma sertonin levels were hightest in the early morning before breakfast and gradually fell to trough levels within 2 h after lunch. The second peak was in the evening before dinner and therafter there were nadir throughout the night time. Oscillations with a period of 1-2 h in the plasma serotonin levels were observed throughout a day. The first peak in the early morning was blunted in obese subjects. Unscheduled food ingestion in the afternoon after a prolonged fast made extended nadir in plasma serotonin levels and the second peak in the evening disappeared. Conclusion: Plasma serotonin levels have a meal-related daily variation, suggesting a role for peripheral serotonin in metabolic regulation.