Figure 1: A photomicrograph of lung sections from (a) a control group showing normal epithelization of bronchi and bronchioles with normal alveoli and normal configuration of thymus (H & E X 200), lung sections from (b) animal treated with single exposure of ethyl carbamate showing significant bronchiolar hyperplasia with sub-epithelial edema and rapid conversion to neoplastic cells accompanied with few mitotic activities (H & E X 400), photomicrograph of lung sections of group treated with repeated exposure (c) showing foccal aggregation of neoplastic epithelial cells around the bronchioles and marked pleomorphic and advanced mitotic activities (H & E X 200).