The metabolism of amyloid β peptide (Aβ) in the brain is crucial to the pathogenesis of Alzheimer disease. A body of evidence suggests that Aβ is actively transported from brain parenchyma to blood across the blood-brain barrier (BBB), although the precise mechanism remains unclear. To unravel the cellular and molecular mechanism of Aβ transport across the BBB, we established a new in vitro model of the initial internalization step of Aβ transport using TR-BBB cells, a conditionally immortalized endothelial cell line from rat brain. We show that TR-BBB cells rapidly internalize Aβ through a receptor-mediated mechanism. We also provide evidence that Aβ internalization is mediated by LRP1 (low density lipoprotein receptor-related protein 1), since administration of LRP1 antagonist, receptor-associated protein, neutralizing antibody, or small interference RNAs all reduced Aβ uptake. Despite the requirement of LRP1-dependent internalization, Aβ does not directly bind to LRP1 in an in vitro binding assay. Unlike TR-BBB cells, mouse embryonic fibroblasts endogenously expressing functional LRP1 and exhibiting the authentic LRP1-mediated endocytosis (e.g. of tissue plasminogen activator) did not show rapid Aβ uptake. Based on these data, we propose that the rapid LRP1-dependent internalization of Aβ occurs under the BBB-specific cellular context and that TR-BBB is a useful tool for analyzing the molecular mechanism of the rapid transport of Aβ across BBB.