Genetic and pathologic studies have associated angiotensin-converting enzyme (ACE) with Alzheimer disease. Previously, we and others have reported that ACE degrades in vitro the amyloid β-protein (Aβ), a putative upstream initiator of Alzheimer disease. These studies support the hypothesis that deficiency in ACE-mediated Aβ proteolysis could increase Alzheimer disease risk and raise the question of whether ACE inhibitors, a commonly prescribed class of anti-hypertensive medications, can elevate Aβ levels in vivo. To test this hypothesis, we administered the ACE inhibitor captopril to two lines of APP transgenic mice harboring either low levels of Aβ or high levels of Aβ with associated plaque deposition. In both models, we show that captopril does not affect cerebral Aβ levels in either soluble or insoluble pools. Furthermore, we find no change in plaque deposition or in peripheral Aβ levels. Data from these Alzheimer models suggest that captopril and similar ACE inhibitors do not cause Aβ accumulation in vivo.