Analgesic Tolerance to Repeated Doses of Subcutaneous Morphine

30 healthy volunteers will be enrolled in this 6-session study. The first visit will be a screening/orientation session. The next 5 visits will be (full day) outpatient sessions taking place Monday through Friday during one week. All full day visits will be drug administration sessions with subjects randomized to one of two groups. The primary purpose is to investigate the time course of development of analgesic tolerance (loss of pain relieving effect) to morphine on experimentally produced skin tenderness in healthy volunteers.