Melatonin (N-acetyl-5-methoxi-tryptamine), a well-known animal hormone synthetised by the pineal gland, plays a key role in the circadian rhythm of vertebrates. An exhaustive bibliographical revision of studies on melatonin in plants published since 1990 points to very few studies (around 20), of which only 8 have a clear plant physiological focus. The data presented in this study demonstrate that melatonin plays a physiological role in plant tissues. Melatonin is seen to be a molecule that promotes vegetative growth in etiolated Lupinus albus L. hypocotyls, in a similar way to IAA. The measurements of melatonin and IAA in lupin hypocotyls by high-performance liquid chromatography with electrochemical detection, and their identification by tandem mass spectrometry, point to a different distribution of these molecules in etiolated hypocotyls.