Recent studies in cell cultures have shown that modulating the cholinergic activity can influence the processing and metabolism of amyloid precursor protein (APP). To investigate whether acetylcholinesterase inhibitors (ChEIs) could decrease production of amyloid β-peptide (Aβ) and slow down the accumulation of Aβ also in vivo, we chronically administered metrifonate (100 mg/kg, po), a second-generation ChEI, to 7-month-old doubly transgenic APP+PS1 mice and their non-transgenic littermate controls for 7 months. Behavioral studies, including open field test, T maze, and water maze, were conducted after 6 months treatment with metrifonate, and the mice were sacrificed at the age of 14 months for biochemical and histological analyses. The long-term treatment with metrifonate failed to inhibit the marked overproduction and deposition of Aβ in the APP+PS1 mice; in contrast, it increased both Aβ40 and Aβ42 levels in the hippocampus. However, the Aβ42 to 40 ratio was significantly reduced by the treatment. In addition, the number of amyloid plaques in the hippocampus did not differ between the treatment and the control groups. Tolerance to cholinesterase inhibition might be induced in the mouse brain because the inhibition rate of AChE was attenuated from about 80 to 50% during the experiment in both APP+PS1 and nontransgenic mice. The metrifonate treatment did not affect cognitive testing parameters but reduced swimming speed and locomotor activity in both genotypes. Our results do not support the idea that ChEIs would slow down the progression of amyloid pathology in Alzheimer's disease. © 2002 Elsevier Science (USA).