Purpose: The purpose of this study was to systematically assess the impact of Alzheimer s disease (AD)-associated blood-brain barrier (BBB) alterations on the uptake of therapeutics into the brain. Methods: The brain uptake of probe compounds was measured in 18-20 month old wild type (WT) and triple transgenic (3?TG) AD mice using an in situ transcardiac perfusion technique. These results were mechanistically correlated with immunohistochemical and molecular studies. Results: The brain uptake of the paracellular marker, [14C] sucrose, did not differ between WT and 3?TG mice. The brain uptake of passively diffusing markers, [3H] diazepam and [3H] propranolol, decreased 54-60 in 3?TG mice, consistent with a 33.5 increase in the thickness of the cerebrovascular basement membrane in 3?TG mice. Despite a 42.4 reduction in P-gp expression in isolated brain microvessels from a sub-population of 3?TG mice (relative to WT mice), the brain uptake of P-gp substrates ([3H] digoxin, [3H] loperamide and [3H] verapamil) was not different between genotypes, likely due to a compensatory thickening in the cerebrovascular basement membrane counteracting any reduced efflux of these lipophilic substrates. Conclusion: These studies systematically assessed the impact of AD on BBB drug transport in a relevant animal model, and have demonstrated a reduction in the brain uptake of passively-absorbed molecules in this mouse model of AD.