In this study, the presence of cells in the intimal region of normal adult bovine pulmonary artery (BPA) was examined by analysis of longitudinal sections at the level of light and transmission electron microscopy. In addition, the morphological and immunohistochemical phenotype of these cells as well as the presence of particular extracellular matrix (ECM) components in this region were also determined. Since ECM production and cell proliferation have been demonstrated to be regulated by locally released growth factors such as transforming growth factor beta (TGF), the presence of TGF-1 in this region was also investigated. Our findings reveal the presence of immature or "nonmuscle" cells into the subendothelial space of normal adult BPA. These cells were characterized by the presence of abundant cytoplasmic organelles and scanty microfilaments. Such cells were negative to antibodies against smooth muscle alpha actin (SM ␣-actin), 1E12, and vWf, but not to vimentin. Similar cells have recently been detected in normal adult BPA and canine carotid arteries, but in the medial region. Because of their location in these elastic arteries, the nonmuscle cells are involved not only in the remodeling of the medial region, but also in the neointima or intimal thickening formation by migration from the media to the subendothelial space, where they proliferate and secrete ECM components. However, a limited number of morphological studies and the current investigation describe the presence of scattered nonmuscle cells within the intima of some normal elastic arteries. This would suggest an important role for these resident cells within the intima in normal and pathological processes as well. In addition, our results show the presence, in this region, of TGF-1 and of ECM components that include collagen, elastin, fibronectin, and laminin which are present in normal conditions and during the intima formation in vivo.