A primary pathological feature of Alzheimer's disease is β-amyloid (Aβ)-containing plaques in brain and cerebral vasculature. Reductions in the formation of Aβ peptides by γ-secretase inhibitors may be a viable therapy for reducing Aβ in Alzheimer's disease. Here we report on the effects of two orally active γ-secretase inhibitors. BMS-289948 (4-chloro-N-(2,5-difluorophenyl)-N-((1R)-{4-fluoro-2-[3-(1H-imidazol-1-yl)propyl]phenyl}ethyl)benzenesulfonamide hydrochloride) and BMS-299897 (4-[2-((1R)-1-{[(4-chlorophenyl)sulfonyl]-2,5-difluoroanilino}ethyl)-5-fluorophenyl]butanoic acid) markedly reduced both brain and plasma Aβ_1–40 in APP-YAC mice with ED₅₀ values of 86 and 22mg/kg per os (po), respectively, for BMS-289948, and 30 and 16mg/kg po, respectively, for BMS-299897. Both compounds also dose-dependently increased brain concentrations of APP carboxy-terminal fragments, consistent with inhibition of γ-secretase. BMS-289948 and BMS-299897 (100mg/kg po) reduced brain and plasma Aβ_1–40 rapidly (within 20min) and maximally within 3h. BMS-299897 also dose-dependently reduced cortical, cerebrospinal fluid (CSF), and plasma Aβ in guinea pigs with ED₅₀ values of 30mg/kg intraperitoneally, without affecting CSF levels of α-sAPP. The reductions in cortical Aβ correlated significantly with the reductions in both plasma (r²=0.77) and CSF (r²=0.61) Aβ. The decreases in Aβ were apparent at 3 and 6h post-administration of BMS-299897, but not at 12h. These results demonstrate that BMS-289948 and BMS-299897 are orally bioavailable, functional γ-secretase inhibitors with the ability to markedly reduce Aβ peptide concentrations in APP-YAC transgenic mice and in guinea pigs. These compounds may be useful pharmacologically for examining the effects of reductions in β-amyloid peptides in both animal models and in Alzheimer's disease.