Recent studies in major depression suggest that a pretreatment systolic orthostatic blood pressure (PSOP) fall of greater than or equal to 10 mm Hg in response to changing from a supine to a standing position may predict response to antidepressant treatment in older depressed patients. Because orthostatic blood pressure response is regulated, in part, by central cholinergic and noradrenergic systems, and both are implicated in Alzheimer's disease, PSOP was assessed as a predictor of initial response in Alzheimer's disease outpatients in a treatment protocol with tacrine, a cholinesterase inhibitor. We found that the magnitude of PSOP fall and increasing age each contributed to the prediction of response to tacrine. These results may suggest a relatively greater involvement of other neurotransmitter systems in younger, nonresponding Alzheimer's disease patients, and the results differ from those of a previous study.