Male rats, kept under a lighting condition of 14 hours light and 10 hours dark were subjected to either ad libitum feeding or 50 per cent caloric restriction. At the end of 3 weeks, body weight and serum tryptophan (TRP) and serotonin (5 HT) were determined over a 24 hours period. It was found that the 40 per cent reduction in body weight was accompanied by a 10 per cent and a 39 per cent reduction in 24 hours mean serum TRP and 5 HT levels respectively. The timing of peak TRP and peak 5 HT levels was probably influenced by the timing of food presentation. Reduced food consumption led to lower peak and trough 5 HT levels but had no effect on peak or trough TRP levels.