The sleep of 6 opiate addicts was studied for 11 nights during 3 phases of a chronic morphine cycle. The control phase consisted of 5 consecutive nights before morphine administration. The induction phase consisted of 1 night at 21-36 days after the onset of morphine administration, when the daily dose was 140-220 mg. The stable dose phase consisted of 5 consecutive nights after the subjects had received 240 mg of morphine daily for 8-19 weeks. No sleep could be studied during the withdrawal phase. Sleep was continuously monitored with EEG, EMG and EOG. Chronic morphine produces signs of a small but persistent sleep disturbance: delta sleep (early night) becomes less stable and shifts toward later in the night, waking state increases during the middle of the night, RME sleep (expecially its activated EEG without eye movements) decreases, the RMES cycle increases, and burst of delta activity (with mean duration of 5-6 sec) increase. Although this disturbance persists throughout the night, it is much less than that seen after single doses of morphine in a previous study. With chronic morphine, therefore, partial tolerance develops to the sleep disturbance produced by morphine. The small but persistent nocturnal arousal during chronic morphine contrasts with the sedation seen during chronic methadone. Both opioids produce an increase in delta bursts during chronic administration, which might be an EEG phenomenon specific to chronic opioid intake.