This study represents a novel clinical trial strategy designed to assess both prospective "prophylactic" therapy for psychopathology in Alzheimer's disease (AD) and to assess an approach that may alter several aspects of the pathophysiology of AD, and perhaps result in alteration of clinical progression of illness. Interpretation of these results will be supported by study of relevant biomarkers and imaging data. Valproate was selected because of its possible symptomatic efficacy for agitation in AD, known safety profile in numerous clinical populations, and in view of recent data supporting its neuroprotective potential in AD. The primary hypothesis is that chronic valproate administration to participants with AD who lack agitation and psychosis at baseline will delay the emergence of agitation and/or psychosis. An effect of this nature may have significant public health implications, for instance, by delaying institutionalization.