Abstract Proliferation of respiratory tract epithelium was studied in adult male rats by using continuous infusions of tritiated thymidine to label desoxyribonucleic acid. The turnover time was the same for tracheal and main bronchial epithelium, but was longer for intermediate bronchi and bronchioles. The structure of the tracheal epithelium is discussed, and the results suggest that a basal cell divides to form one basal and one superficial cell, and a superficial cell divides to form two superficial cells (one of which is lost either sequentially or randomly). In 330 g black-hooded rats the turnover time is 82 days for the basal cells and 131 days for the superficial cells if these are lost sequentially. In younger rats with a higher rate of growth the turnover was faster.