Granulomatous lung lesions were produced in rats by 5 intravenous or intratracheal injections of the killed BCG suspension as an antigen. On the basis of immunocomplex and cell-mediated immunopathological reactions this condition may be considered as a chronic allergic form of alveolitis. Granulomas were formed along the arteries when the antigen was administered intravenously and along the bronchi after its intratracheal administration. In both cases there were single granulomas in the alveolar septa interstitium. The microcirculatory bed was studied in semithin sections and morphometrically in thick sections after impregnation with an Indian ink and 5% gelatin. The alveolar septa capillaries in control rats were shown to form a network the parameters ow which rather correspond to the "sheet-blow" model (Fung and Sobin, 1969). In chronic allergic alveolitis, a reduction of the capillary network develops at the zone of granulomatous inflammation while in the relatively normal zone a network type of the microcirculatory bed is preformed into the main one with the formation of predominant routes of circulation that provide the blood filling of the heart cavities.