Inhaled particulates which stimulate a 'delayed', cellular mode of alveolar clearance are excreted to the airways through lymphoid foci in the bronchial bifurcations. The anatomic relations and developing pathology of the tissues adjacent to these foci, including the divisions of accompanying arteries, were studied by serial sectioning and photomicrographic modelling of rat lungs. The changes are typical of classic 'delayed' inflammatory reactions and, in the rat, the fully developed stage is characterised by fibrinoid necrosis involving all three layers of the arterial wall in a linear lesion across the leading edge of the flow divider. An hypothesis was developed to relate the injury to pulsatile forces. Recent published findings indicate that similarly placed lesions, with species-specific changes in development, are universal in both cerebral and extra-cranial arterial forks of man and animals. Possible associations of the microvascular changes with human atherosclerosis and their further significance in pulmonary and systemic effects arising from industrial and environmental contaminants are explored.