Male Syrian hamsters maintained on a 14-h light, 10-h dark photoperiod were injected once daily (1-2 h before lights out) with melatonin (25 micrograms), once every other day with T4 (5 micrograms every other day), or melatonin every day plus T4 every other day. Hamsters which received melatonin injections, with or without T4, had serum levels of insulin-like growth factor that were increased 2-fold. The results suggest that melatonin-induced changes in body weight are a result of long-term changes in insulin-like growth factor-I secretion.