Abstract. Food restriction (50%) effects on the 24-h rhythm of serum and pineal melatonin (MT) were studied in 260–300 g male Wistar rats under a lighting regimen of 14 h light and 10 h dark. Body weight, testicular weight, accessory organ weights, serum LH, serum testosterone, and 24-h rhythms of serum and pineal MT were determined. One week of food restriction caused a decrease in body weight (18%), accessory organ weights (18%), and serum LH (50%), but had no effect on serum or pineal MT. Three weeks of food restriction suppressed the body weight and accessory organ weights further (35% and 39%, respectively), reduced serum LH (68%) and serum testosterone (53%), reduced pineal MT (12%) and raised serum MT (34%). The increased serum MT may play a role in the reported potentiation of pineal action in food deprived rats.