Abstract
The histomorphology of the terminal bronchiolar region of the mouse, rat, hamster, guineapig, gerbil and rabbit was studied. Although some general structural similarities existed in the progressive intrapulmonary branching pattern of the airway tree between species, there were conflicting accounts in the literature about the presence of the respiratory bronchioles in common laboratory mammals. In our light microscopy study we failed to detect the existence of typical respiratory bronchioles with characteristic interruptions on their walls projecting into the alveoli. Frequently in these species the terminal bronchioles were short and abrupt, opening directly into several alveolar ductules.