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Using Yanagita's activity-level counter, effects of morphine administration upon the spontaneous activities of rats under various conditions were studied. And the following facts were elucidated;1. It was shown numerically that the more the size of the dose was large and the more the duration of morphine repetition was long, the faster and the stronger their spontaneous activities became.2. Such changes of the spontaneous activities of rats due to the repeated administration of morphine can be regarded as one the signs of tolerance.3. The changes of the spontaneous activities of rats or some sorts of tolerance occur as fast as the appearance of physical dependence.4. The mechanisms by which rats become hyperactive by the repeated administration of morphine may be explained chiefly by the development of tolerance to the sedative action of morphine and by the absence of tolerance to the excitatory action of the drug. However, the extraordinary increases recognized by the long term repetition of morphine request us further studies.5. As far as slow morphinization of rats is adopted, the abstinent phenomenon of the morphinized rats is sedation.6. The disappearance of tolerance after the cessation of morphine administration occurs very late, far later than the disappearance of abstinent phenomenon.7. Frequent administrations of nalorphine to the morphinized rats can not shorten the duration of tolerance retention after the cessation of the medication.