Following monocytic depletion, alveolar macrophages are produced by a local cell renewal system within the pulmonary interstitium. The contributions of hematopoietic and interstitial components to the output of macrophages in the normal state is not known. In the present autoradiographic study, sequential labeling and grain counts in monocytes, interstitial cells, and free alveolar macrophages are examined in mice up to 7 weeks after three injections of 3H-thymidine; the animals also received colchicine 4 hours before sacrifice. The labeling patterns of interstitial cells and macrophages are biphasic. The initial peaks with very few mitotic figures are consistent with migration of blood monocytes across the interstitium to the alveoli. In the second phase, when no labeled monocytes are present, a few heavily labeled macrophages are observed up to 7 weeks; in this period mitoses and heavy labeling are seen in a few interstitial mononuclear cells. It appears that, under normal circumstances, the predominant mechanism of macrophage production is direct passage of monocytes across the interstitium to the alveoli. This may be rapid at the thin walled capillaries and take up to 2 days where the interstitium is thicker. A smaller proportion of cells appears to arise by division of resident interstitial cells with subsequent migration to the alveoli. The sojourn of free macrophages in the alveolus under basal conditions is 7 days.