The lung of the tortoise, Testudo graeca (Chelonia) was studied by means of light and electron microscopy, histochemistry and immunocytochemistry. The lung showed the typical faviform structure of the reptilian lung. Three orders of trabeculae were observed. The epithelium of primary and secondary trabeculae was composed of ciliated, mucous, basal and endocrine cells. Mucous cells contained sialo- and sulpho-mucins and were reactive to the lectins Con-A, WGA, DBA, PNA and SBA. Endocrine cells were observed as solitary cells or forming neuroepithelial bodies. By means of immunocytochemistry, endocrine cells were demonstrated to contain serotonin. In the gas-exchange area Types I and II pneumonocytes and undifferentiated cells were observed. Free macrophages were detected in the faveolar lumen. The lung interstitium contained smooth muscle cells, fibrocytes, pigment cells, myelinated and unmyelinated nerves and intrapulmonary ganglia. Nerve terminals containing clear and dense-cored vesicles were observed in the adventitia of the blood vesicles and interspersed between the smooth muscle bands. The lung of the hibernating specimens showed a marked vacuolisation of pneumonocytes. In conclusion, the lung of Testudo graeca showed a complex histological organisation. Marked differences from mammalian lung were found.