In previous clinical studies of post-surgical pain the investigators found that nalbuphine (Nubain), a narcotic pain killer, relieves pain more effectively when combined with low-dose naloxone (Narcan), a drug that is used to treat narcotic overdose. This finding was particularly true in men. The purpose of this study is to find out if nalbuphine combined with naloxone is more effective in relieving experimentally produced pain than either drug alone. A second reason for this study is to find out if study medications work more effectively in women or in men. Subjects will come to the University of California at San Francisco (UCSF) Clinical Research Center (CRC) for 4 study visits. The first visit will be a 2-hour screening to assess the subject for study eligibility. During the other three visits, the investigators will use a thermal stimulating device to produce temporary, non-injurious skin sensitivity that subjects will feel as painful. Changes in pain will be measured following the intravenous (i.v.) administration of study drugs. Three drug combinations will be administered, a different one each visit: 1) nalbuphine 5 mg and naloxone 0.4 mg , 2) naloxone 0.4 mg and saline (an inactive solution), nalbuphine 5 mg and saline. These drug combinations will be administered in random order; all subjects will receive all three combinations.