The histo-autoradiographic distribution of labelled Almitrine was quantitatively assessed on semi-fine cuts of rat lung. The experimental set-up consisted of an isolated heart-lung preparation, perfused for two hours with homologous blood containing 10 microCi per ml of H3 almitrine (990 mCi/mM) et 0.1 microCi per ml of C14 almitrine (9.9 mCi/MM). Sequential histological biopsies were made. There was an apparent heterogeneous radioactivity of the tissues overall with no observed increase in fixation with perfusion time (v.i.). Nevertheless the auto-radiograph enabled this conclusion to be rectified by revealing the cause of the heterogeneity, the embolisation of micro-aggregates of the labelled product. After correction was made for this factor the scattering of the fixed radioactivity in the samples was weak. The localisation of the labelled product showed a good stability for the fixed marker in the tissues, and identified the distribution of the tracers H3 and C14, a preferential fixation by alveolar macrophages, type-2 pneumocytes, Clara cells and adipocytes. A remarkable fixation was noted in the cardiac fibres of the pulmonary veins. Amongst the cells of the bronchi, the mucus cells were notably active for the fixation of Almitrine. There was no detectable fixation associated with the nervous tissues and the neuro-epithelial body was not labelled. A sequential analysis showed that only fixation in smooth muscle fibres rose significantly with perfusion time.