Development of rational therapeutic treatments of Alzheimer disease (AD) requires the elucidation of the etiopathogenic mechanisms of neurofibrillary degeneration and β-amyloidosis, the two hallmarks of this disease. Here we show, employing an adeno-associated virus serotype 1 (AAV1)-induced expression of the C-terminal fragment (I(2CTF)) of I(2)(PP2A), also called SET, in rat brain, decrease in protein phosphatase 2A (PP2A) activity, abnormal hyperphosphorylation of tau, and neurodegeneration; littermates treated identically but with vector only, i.e., AAV1-enhanced green fluorescent protein (GFP), served as a control. Furthermore, there was an increase in the level of activated glycogen synthase kinase-3β and enhanced expression of intraneuronal Aβ in AAV1-I(2CTF) animals. Morris water maze behavioral test revealed that infection with AAV1-I(2CTF) induced spatial reference memory and memory consolidation deficits and a decrease in the brain level of pSer133-CREB. These findings suggest a novel etiopathogenic mechanism of AD, which is initiated by the cleavage of I(2)(PP2A), producing I(2CTF), and describe a novel disease-relevant nontransgenic animal model of AD.