 - To determine whether low does of the α2A-adrenoceptor agonist guanfacine can improve deficits in prefrontally-mediated working memory and executive control functions, in healthy elderly subjects. Subjects will be randomly assigned to receive placebo or guanfacine at one of two dose levels: 0.1 mg, or 0.5 mg daily at bedtime in double-blind fashion for twelve weeks. 