The purpose of this study is to conduct experiments to examine subjective and reinforcing effects of nitrous oxide. Mood altering and psychomotor effects will be tested on non-drug abusers and preference procedures will be used to assess reinforcing effects. Comparisons between nitrous oxide, opiates, and benzodiazepine antagonists will be made. To examine sevoflurane versus isoflurane inhalation at subanesthetic concentrations on mood, pain, and psychomotor performance.