Melatonin (N-acetyl-5-methoxytryptamine) is a highly conserved molecule whose presence is not exclusive to the animal kingdom. Indeed, numerous studies have demonstrated its presence in plants, where the possible role(s) of this indoleamine is (are) under active investigation. The present work aims to further our knowledge in this respect and presents the results of a study of the effect that melatonin has on foliar senescence. Barley leaves treated with melatonin solutions clearly slowed down the senescence process, as estimated from the chlorophyll lost in leaves. This effect of melatonin was concentration dependent, with an optimal response being obtained at 1 mm melatonin, after 48 hr of incubation in darkness. The already known effects of the phytohormones, kinetin, and abscisic acid, were also assayed. Of the phytohormone and melatonin combinations assayed, 1 mm melatonin presented the best protection against senescence. The levels of endogenous melatonin in control leaves were measured by liquid chromatography with fluorescence detection and in leaves treated with different exogenous melatonin concentrations (to demonstrate the absorption capacity of leaves). The possible physiological implications of this newly revealed action of melatonin in foliar senescence are discussed.