The indoleamine melatonin, a well-known animal chemical, has been identified in extracts from several plant species. The function of melatonin in plants is unknown. Two major functions of melatonin in animals are dark signaling and antioxidant protection. Fruit ripening was used as a model physiological process that involves changes in the oxidative status of an organ. Tomato fruits at various stages of ripeness were sampled. Morning glory (Pharbitis nil Choisy, cv. Violet) and tomato (Lycopersicon esculentum Mill., cv. T5 and Castlemart) organs were collected throughout a light/dark cycle to determine whether melatonin levels increased during the night. No consistent evidence was found that melatonin increased significantly in organs of these plants during the night, as it does in many animals. The melatonin content of the fruits generally increased during ripening up to the mature ripe stage and thereafter as the fruit became over ripe.