Tau is one of the major microtubule-associated proteins in neuron. The abnormal hyperphosphorylation of tau is believed to be critical for pathogenesis of Alzheimer disease. The autopsied human brain tissue is commonly available with longer than 2~3 h of postmortem delay. Therefore, understanding whether and how postmortem delay affects tau phosphorylation is very important for studies of the function of tau and the role of tau phosphorylation in AD pathogenesis. Tau phosphorylation in normal rat brain and its change during postmortem delay were measured by using site-specific and phosphorylation dependent anti-tau antibodies. It was found that tau in normal rat brain was phosphorylated with different extents at Thr181, Ser199, Ser202, Thr205, Thr212, Ser214, Thr217, Ser396, and Ser404, but not at Ser262, Ser409, and Ser422. Tau was rapidly dephosphorylated at all of the sites measured in 3 h after death and the dephosphorylation continued till 6 h postmortem delay although total tau level did not change during this period of time. The activities of both tau kinase and tau phosphatase in rat brain were found decreased upon temperatures, and the kinase activity was more sensitive to temperature than phosphatases activity at 25℃. The rapid dephosphorylation of tau after animal death was due to the relative higher activity of tau phosphatase in the brain.