The circadian plasma melatonin profile of a marsupial, the tammar, was determined at various stages of the annual reproductive cycle. At 6-14 days after each of the solstices and equinoxes, six females were exposed to a photoperiod equivalent to the natural day length at these times. Serial blood samples were taken 8 days later at 2-4-hourly intervals, and plasma melatonin concentrations were measured by radioimmunoassay. Melatonin concentrations were elevated during the dark phase of each photoperiod, and there were significant changes between the profiles in each season. The amplitude of the nocturnal rise was significantly higher (P less than 0.05) during the breeding season after the summer solstice (peak 259.5 +/- 26.8 pg/ml, mean +/- SEM) and autumnal equinox (287 +/- 53.2 pg/ml) compared to those during the nonbreeding season after the winter solstice (111.5 +/- 10.5 pg/ml) and vernal equinox (154.5 +/- 10.4 pg/ml). The duration of the nocturnal rise was significantly correlated (r=0.996, P less than 0.01) with the length of the dark phase and so was shortest after the summer solstice and longest after the winter solstice. Either of these changes in amplitude or duration might provide the photoperiodic information that regulates the annual reproductive cycle of the tammar.