Short-day-induced fattening was investigated in the small nocturnal primate Microcebus murinus. Animals were either transferred from long photoperiod (LP) to short photoperiod (SP) or maintained in LP and submitted to various treatments. Animals fed daily on 40 microg of melatonin (MEL) at two different times showed a significant increase in body mass compared to LP-exposed animals. This weight gain was more marked in animals fed with MEL 2 h before night onset, suggesting a critical period around the first hours of the night. The MEL-induced fattening was associated with an increase in food intake and a decrease in plasma thyroxin (T4) levels. On the other hand, a decrease in T4 plasma levels induced by two doses of methimazole (MET) led to either an increase (3 mg/day) or a decrease (10 mg/day) of body mass, suggesting a role of thyroid hormones in the photoperiodic regulation of body mass. The MET-induced fattening did not involve any change in food intake. These findings suggest that autumn fattening in the gray mouse lemur proceeds from two distinct mechanisms. First, an increase of food intake may result in the SP-induced increase in MEL secretion. Second, a decrease of energy expenditure may be due to the MEL-induced hypothyroidism.