Large bilateral preoptic lesions produced complete sleeplessness in two cats. In eight additional cats, similar but smaller lesions resulted in a significant reduction of quiet (slow-wave) sleep by 55 to 73 percent, and active (paradoxical) sleep by 80 to 100 percent. These values were determined by pre-and postlesion 22-hour continuous observations. Complete sleeplessness was followed by lethal exhaustion within a few days, whereas incomplete sleeplessness persisted at maximum levels for 2 to 3 weeks. The suppression of sleep was characterized by a gradual onset during the first 1 to 2 weeks, and a complete or partial recovery after 6 to 8 weeks. The severity of sleep suppression was found to be related to the size and localization of lesions placed specifically within the preoptic area and not to transient disturbances in feeding and temperature regulation.