Morphine pellets (2 x 75 mg) were subcutaneously implanted in rats and vigilance states (wakefulness, slow wave sleep and paradoxical sleep) were observed during ten days. Significant impairment of each vigilance state distribution appeared during the first days of morphine dependence. Although waking and slow wave sleep were not affected during the last days, paradoxical sleep duration was reduced during dependence. Nevertheless, the sleep-wake circadian rhythm was not abolished. These results suggest that the sleep process is affected differently in its components (slow wave sleep and paradoxical sleep) during morphine dependence.