The phospho-tau epitope Thr231 is immunohistochemically detectable in 3xTg-AD mouse hippocampus by 6 months of age. Coronal mouse brain sections (30 μm) were prepared from 3xTg-AD mice sacrificed at 2 (A, I, Q), 3 (B, J, R), 6 (C, K, S), 9 (D, L, T), 12 (E, M, U), 15 (F, N, V), 18 (G, O, W), and 26 months of age (H, P, X) and were processed for immunohistochemistry using the AT180 monoclonal antibody to detect phospho-tau (Thr231) expression. CA1 hippocampal sections at Bregma -1.8 mm (A–H), at Bregma -2.5 mm (I–P), and at Bregma -2.8 mm (Q–X), were examined for regional and temporal patterns of human phospho-tau and photomicrographs were obtained at 10×. The inset in panel X represents a 40× digitally magnified image of the photomicrograph for better visualization of stained cell morphology. Scale bar in D represents 200 μm.