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Nitric oxide synthase (NOS) produces nitric oxide, a mediator ofpotential importance in numerous physiologic and inflammatory processes in the lung. We localized constitutive NOS (c-NOS) and inducible NOS (i-NOS) within lung tissue by immunoperoxidase labeling with specific antibodies or byhistochemical demonstration of the characteristic NADPH diaphorase activity of NOS. We analyzed human airway (n= 4) or parenchyma (n= 10) specimens obtained from uninvolved areas of surgical tumor resections. We also studied human fetal lung samples (n= 6) and normal or inflamed (16 h after intratracheal LPS instillation) rat lung tissue. lmmunostaining with anti-c-NOS identified c-NOS antigen in rat lung nerves, endothelium, and airway epithelium. Normal or inflamed rat macrophages were not stained. Human nerve elements and large-vessel endothelium showed immunostaining with the anti-c-NOS, but no …