Dysregulation of the opioid system may underlie the pathophysiology of mood disorders, such as bipolar disorder. Drugs that modulate the opioid system might be effective treatments for bipolar disorder. The profile and actions of the kappa-opioid system make drugs that target this system particularly promising as a treatment modality, with relatively low risk of addictive properties. Pentazocine is an approved drug for pain relief with a good side effect profile. It is predominantly a kappa opioid agonist with weaker side effects at mu opioid receptors, at which it is an antagonist. Data from our open-label pilot study of pentazocine had promising results. We will follow up on these findings with a double-blind, active-control study of individuals with bipolar disorder or schizoaffective disorder who are currently hospitalized with acute mania. The antimanic effects of pentazocine will be compared with an active control (ativan).