The aim of these experiments was to determine the contribution of leukocyte‐derived iNOS to total iNOS expression induced by lipopolysaccharide (LPS). By transferring bone marrow between iNOS^+/+ and iNOS^–/– mice, we created chimeric mice in which iNOS expression was limited to either circulating leukocytes (leukocyte‐iNOS mice) or parenchymal cells (parenchyma‐iNOS mice). Analysis of congenic markers demonstrated that >95% of thymocytes in chimeric mice were of donor origin. Also, following LPS treatment, iNOS mRNA was detectable in blood from leukocyte‐iNOS mice but not parenchyma‐iNOS mice. Together these findings indicated that the host marrow had been replaced entirely by donor cells. In the lung, at least 50% of the LPS‐induced iNOS mRNA was derived from leukocytes, and immunohistochemical analysis indicated that leukocytes were the main source of iNOS protein. In contrast in the liver, colon, and muscle, iNOS expression was derived predominantly from parenchymal cells. This divergence is potentially explained by the high level of leukocyte recruitment to the lung, relative to the other tissues. Plasma levels of NOS byproducts indicated that parenchymal iNOS was the dominant source of systemic iNOS activity. These findings indicate that in tissues other than the lung, parenchymal cells are the principal source of iNOS during endotoxemia.