Growing male domestic fowl of an egg-laying strain were fed ad libitum and injected intraperitoneally with melatonin or intramuscularly with triiodothyronine (T3) to study the effects on sleep, food intake, blood glucose, e.e.g., oxygen consumption and carbon dioxide production. Melatonin caused a dose-related depression of food intake with sleep and aphagia lasting for 2 1/2 hr following 8 mg, drowsiness and greatly reduced intake following 4 and 2 mg and a slight reduction in food intake after 1 mg. T3 injection was followed by increased feeding within the range 50-200 micrograms. The higher dose (200 micrograms) completely prevented the effects of 10 mg melatonin injected simultaneously. Melatonin (10 mg) depressed plasma glucose levels whereas T3 (200 micrograms) elevated blood glucose. Either darkness or melatonin (10 mg) caused an increase in amplitude and a decrease in frequency of the e.e.g. Birds fasted for 3 hr before injection showed significantly lower oxygen consumption and carbon dioxide production when given melatonin (10 mg); T3 had no effect within the 4 hr period after injection and did not modify the effects of melatonin. It is postulated that the rapid effects of melatonin and T3 which were observed result from direct effects of these hormones on the central nervous system.