Morphological changes of rat alveolar type II (AT-II) cells were studied at 8 and 10 months following inhalation of 239PuO2 to elucidate the biological role of AT-II cells in the induction of lung tumours. TEM micrographs of random sections of lung were analysed qualitatively and quantitatively using an automatic image analyser. Eighteen morphometric parameters were obtained according to stereological principles. The results showed that, following the inhalation of 239PuO2, AT-II cells became less differentiated and the metabolism of the pulmonary surfactant in AT-II cells was disturbed.