Melatonin is an animal hormone that regulates photoperiodic reactions in many vertebrates. Its level is usually higher at night, and the duration of elevated melatonin levels is proportional to the length of the night, thus giving information about day length. We have previously identified melatonin in the dicotyledonous plant Chenopodium rubrum. We compared melatonin daily profiles in C. rubrum shoots when given three different photoperiodic regimes with 8 h, 12 h, and 16 h of darkness. As in vertebrates, melatonin levels reached its maximum at night, but the duration of elevated melatonin content did not change with the photoperiod. The maximum melatonin content occurred at later times after lights off as night length increased, but always 6 h before lights on in all light/dark cycles. These results indicate differences in the regulation of melatonin levels between vertebrates and higher plants. We suggest that the timing of melatonin maximum relative to lights off may provide photoperiodic information for C. rubrum.