The aim of the study was to evaluate the quantitative and morphological changes in type II alveolar epithelial cells in the course of cyclophosphamide-induced lung damage. The experiments used 40 Wistar rats, of 170 g body weight. The animals were divided into two experimental groups. Group I animals were given cyclophosphamide (Endoxan-ASTA) in a single intraperitoneal dose of 150 mg/1kg b.w./1 ml PBS. Group II (control) received 1 ml PBS. All the animals were sacrificed after 1, 7, 14 and 28 days following intraperitoneal cyclophosphamide or PBS administration. Morphological examinations of pulmonary tissue were based on ultrastructural analysis in the transmission electron microscope. Quantitative studies of type II cells were performed basing on sections stained for alkaline phosphatase. The results of the quantitative analysis showed statistically significant alterations in the number of type II alveolar epithelial cells after 14 and 28 days following cyclophosphamide administration, compared with the respective control groups. The increase in the number of type II cells was accompanied by the intensification of fibroplasia processes within the interstitium of the interalveolar septa of the lungs. Ultrastructural exponents of active participation of type II cells in fibroplasia processes were found.