ABSTRACT. Bipolar Disorder (BD), defined by a history of hypo/mania, is common and debilitating. Yet, the neural mechanisms predisposing to hypo/mania, to guide new interventions for BD, are poorly understood. BD is characterized by abnormally elevated reward sensitivity, impulsivity and sensation seeking, response tendencies that predispose to hypo/mania in potentially rewarding contexts, e.g., uncertain reward expectancy (RE). In adults with BD, we reported abnormally elevated uncertain RE-related left ventrolateral prefrontal cortical (vlPFC) activity. Moreover, we showed a positive relationship between RE-related left vlPFC activity and an impulsivity component, negative urgency; and that negative urgency mediates a positive association between RE-related left vlPFC activity and the severity of lifetime predisposition to hypo/mania in young adults who have not yet developed BD. Abnormally elevated left vlPFC activity to RE is thus a potential neural mechanism underlying heightened negative urgency, which confers risk for development of/ worsening hypo/mania. Theta- burst stimulation (TBS) is a Repetitive Transcranial Magnetic Stimulation (rTMS) paradigm that can acutely, rapidly, and non-invasively modulate the left vlPFC. Examining if continuous (inhibitory) TBS (cTBS) over left vlPFC leads to acute changes in hypo/mania-related affect is thus a first step toward elucidating the neural mechanisms that predispose to hypo/mania. We will recruit 50 remitted/mild-moderate hypomanic adults with BD type I (in whom we reported the majority of RE-related neuroimaging data): 18-35 yrs (unmedicated/ on common BD medications), to avoid confounds of long psychiatric illness/ long medication history; and 50 age- and gender ratio-matched healthy/ non BD (history of anxiety/ non BD mood disorders) adults. We will examine activity in and functional connectivity (FC) among left vlPFC and reward regions: ventral striatum (VS), amygdala, orbitofrontal cortex (OFC) and dorsal/rostral anterior cingulate cortex (d/rACC). Each participant will have baseline assessments of negative urgency (and other BD-related response tendencies) and a baseline structural scan for neurotargeting and TBS dose thresholding. One week later, there will be 3 scan sessions over <2 weeks: each with 1 of 3 TBS conditions interleaved between pre and post cTBS scans, in randomized order: left vlPFC cTBS; left control region, somatosensory cortex, cTBS; and left vlPFC sham TBS. Positive and negative affect will be measured before each pre cTBS and after each post cTBS scan. We aim to: 1. Determine the impact of acute cTBS over left vlPFC (vs. other cTBS conditions) on RE-related activity in and FC among left vlPFC, VS, amygdala, d/rACC, OFC; 2. Determine if cTBS-induced neural changes lead to acute changes in hypo/mania- related affect, and if negative urgency moderates these relationships; 3. Compare effects of cTBS over left vlPFC (vs. other conditions) on neural-affect measures in BD vs....