Adult female Sprague-Dawley rats were given 10 mg/kg i.p. injections of morphine sulfate twice weekly, at 84-h intervals, over a period of three weeks. A control group of rats was simultaneously treated with equivalent volumes of isotonic saline. The animals were then prepared with cortical and temporalis muscle electrodes. Ten days after the last injection of morphine or of saline, they were placed in individual cages for recording of the EEG and the EMG and both groups were given i.p. a 10 mg/kg test dose of morphine. In the saline-treated rats, high voltage EEG slow bursts in association with stuporous behavior appeared almost immediately after injection and prevailed for 60–90 min. This phase was followed by continuous EEG and behavioral arousal for another period of 60–90 min, after which sleep appeared. Administration of the 10 mg/kg test dose of morphine to the rats having prior morphine exposure resulted in a much shorter initial period of EEG and behavioral stupor and a longer secondary phase of EEG and behavioral arousal. The duration of the entire morphine effect as determined by the latency to sleep onset, however, was the same in the saline-treated and morphine-treated groups of rats. These results support the assumption that long-term alterations in the function of the CNS occur not only after morphine addiction, but also after only exposure to morphine.