Melatonin (N-acetyl-5-methoxytryptamine) is an animal hormone synthesized predominantly at night. It often serves as a signal of darkness that regulates circadian rhythmicity and photoperiodism. Melatonin has also been found in algae and higher plants, including the short-day flowering plant Chenopodium rubrum. To test its involvement in plant photoperiodism, melatonin solutions were applied to the cotyledons and plumules of 5-day-old-seedlings of Chenopodium rubrum L., ecotype 374. 3H-labelled melatonin was readily taken up by the plants and was very stable for a period of 37 h from application. Treatment with 100 and 500 µM melatonin significantly reduced flowering of plants exposed to a single inductive 12-h darkness. Melatonin was efficient only when applied before lights off or during the first half of the dark period. This indicates that melatonin affects some early steps of the transition to flowering. However, it had no effect on the period or phase of a circadian rhythm in photoperiodic time measurement. Melatonin agonists (2-I-melatonin, 6-Cl-melatonin, CGP 52608) and 5-hydroxytryptamine also reduced flowering, whereas 5-methoxytryptamine did not. The results demonstrate that exogenous melatonin is able to influence the early stages of photoperiodic flower induction and/or flower development in a higher plant. Possible mechanisms for this effect are discussed.