Morphine tolerance and sensitization in the hamster.

The effects of morphine on hamster locomotor activity were studied. Repeated administration of morphine in doses from 5 to 40 mg/kg produced systematic changes in morphine's biphasic time effect pattern: morphine's sedative effects decreased (tolerance) while morphine's excitatory effects increased (sensitization). These results extend findings of behavioral tolerance and sensitization in the hamster to a range of higher doses than those used previously.