The β-amyloid protein (βA4) is released by proteolytic cleavages from the Alzheimer amyloid protein precursor (APP), which is an integral membrane protein modified by phosphorylation on its cytoplasmic domain. Phosphorylation of βAPP is a physiological event in brain tissues, although functions of phosphorylation are still unknown.In the present study, we developed a phosphorylation state-specific antibody to phospho-Thr668 site (numbering for APP 6 9 5 isoform); a site phosphorylated by cdc2 kinase in cultured cells, to analyze the phosphorylation state of APP in rat tissues. By utilizing this antibody and anti-APP antibody which does not show cross-reactivity to APLP2, we demonstrated that Thr668 is phosphorylated specifically in brain tissues and the phosphorylation level at the site in tissues other than brain was below detectable levels although APP is expressed in all tissues examined. Furthermore, immunohistochemical studies in rat brain revealed that the phosphorylated form of APP exists only in neuropil, although APP exists in somata as well as neuropil. Our data suggest a possibility that phosphorylation of APP plays an important role in brain specific functions and/or metabolism of APP.