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. Group A will receive two daily intramuscular injections of 8 mg morphine on Monday through Thursday and two intramuscular injections of placebo on Friday. Group B will receive two daily intramuscular injections of placebo on Monday through Thursday and two intramuscular injections of 8 mg morphine on Friday. During the Monday through Friday sessions the painfulness of experimental heat application and the area of skin tenderness (primary and secondary hyperalgesia) will be assessed before and two times after the intramuscular injections. Heat will be applied by using a computer controlled probe that applies heat (450 C/1130 F) on the thigh for 3 minutes (Brief Thermal Sensitization model.)