Djungarian hamsters (Phodopus sungorus) kept under a long-day photoperiod (16 h light :8 h dark) were injected with melatonin each day. Hamsters which responded physiologically to this treatment (gonadal regression, molt, body weight loss) phase-advanced onset and extended duration of activity. Hamsters which were physiologically insensitive to melatonin injections did not exhibit such changes in activity pattern and often failed to entrain to the light :dark cycle. Hamsters given saline injections did not alter activity or exhibit gonadal regression, weight loss and molt to the winter pelt. Melatonin-sensitive hamsters compressed duration of activity when they became physiologically refractory to the melatonin treatment (weeks 27–29). At the same time, melatonin-insensitive hamsters became entrained to the light :dark cycle. Thus, daily melatonin injections induce short-day-like adjustments in activity under a long-day photoperiod. These changes in activity are correlated with melatonin-induced gonadal regression, weight loss and mol