Daily infusions of melatonin restore sleep suppressed by continuous bright light in pigeons. To test whether melatonin could also induce sleep in pigeons on a 1212 h light-dark cycle (LD), pigeons received 12-h intravenous melatonin infusions during the day. Melatonin induced sleep during the day, increased EEG slow wave activity, and decreased body temperature and locomotor activity. None of these variables were altered during the night following infusions. The induction of extended daytime sleep by melatonin infusions indicates that melatonin is a principal factor in the regulation of sleep in pigeons.