Photomicrographs of typical lung lesions in NNK-treated Wistar rats. (A) Alveolar epithelial hyperplasia in a Wistar rat (magnification × 200). (B) Dysplasia occurred in the alveolar region. Some of the alveolar structure was absent and replaced by proliferated cells (magnification × 200). (C) Proliferated cells with increased cellular atypia (magnification × 400).