Five cats were prepared with chronically implanted electrodes for recording sleep-wakefulness patterns. Four of these animals received fenfluramine at each of three dose levels and data was recorded for the following 12 hours. Percent of time in paradoxical sleep was significantly reduced by 2.5 and 7.5 mg/kg, but not by 0.5 mg/kg, of fenfluramine. The higher doses also increased slow-wave sleep and, at 7.5 mg/kg (an anorexigenic dose), total sleep time was significantly increased. Under similar conditions amphetamine, at an anorexigenic dose of 1 mg/kg, significantly suppressed both paradoxical sleep and slow-wave sleep in three cats.