The opiate neurotransmitter system is thought to be involved in many abnormal mood states. Some researchers have suggested that changes in this system may trigger the switch to/from manic and depressive states in bipolar disorder. One problem with most of the currently available opiate medications is that they can produce addiction/dependence. A particular kind of opiate medication known as kappa-opiates may be able to produce changes in this system with much less risk of addiction. This study looks at Talwin (a combination of pentazocine and naloxone), a medication which affects the kappa and mu opiate systems. The study will examine whether two doses of Talwin affect manic symptoms in people who have been admitted to the hospital. This study will give more information about the involvement of the opiate system in bipolar disorder, and give important information for use in developing new treatments.