The purpose of this trial is to demonstrate whether valproate therapy delays the emergence of agitation and/or psychosis in outpatients with probable Alzheimer's disease (AD) who have not experienced agitation and psychosis in their illness. A secondary aim is to determine whether valproate therapy delays the progression of cognitive and functional measures of the illness. This trial will also assess the tolerability and safety of low-dose, long-term valproate therapy. Valproate, an anticonvulsant drug, 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.