The purpose of this study was to follow the evolution of normal and tested rat embryo lungs in organ culture and to classify the induced lesions in relation to the in vivo tumour histogenesis. Lungs from 15 day old Wistar rat embryos were maintained in organ culture in M 199 medium and 20% horse serum and treated with benzo[a]pyrene (B[a]P) or cigarette smoke condensate (CSC). Explants were studied 30 days of culture. The control explants display a prevalence of epithelial tissue along with a reduction in connective tissue, whereas treated explants show similar morphological alterations after both B[a]P and CSC. The percentage of the observed lesions depends on the strain of rats used. Alterations are classified into preneoplastic stages according to Shabad's pattern developed for rat lung tumours in vivo. This classification permits the assessment and quantitation of the carcinogenic effect of a substance. To prove that lung explants may undergo neoplastic transformation after treatment in vitro, normal and carcinogen-treated lung organ explants were dispersed and, after cell subculture, injected i.p. into isogenic rats. Tumours were obtained after only 5 subcultures with treated cells, whilst with the controls, tumours were obtained after 21 subcultures.