We investigated the effects of a single administration of a cholinesterase inhibitor, tetrahydroaminoacridine (THA, 25 and 50 mg, orally), and an alpha 2-agonist, clonidine (0.5 and 2 micrograms/kg, orally), on neuropsychologic performance in two groups of patients with Alzheimer's disease (AD). Clonidine enhanced a spatial working memory and verbal fluency, but had no effect on spatial span or word priming. THA enhanced word priming, but had no effect on other performance measures. Our data suggests that degeneration of the LC noradrenergic system and the cholinergic cells of the basal forebrain have different functional consequences during the progression of AD. Finally, a combined treatment with noradrenergic and cholinergic drugs might produce a qualitatively broader effect on cognitive functions than either of the treatments alone, and more effectively attenuate clinical dementia.