A previously unknown rod-shaped cytoplasmic component, consisting of a bundle of fine tubules enclosed by a tightly fitting membrane, was regularly found in endothelial cells of small arteries in rats and humans. These components are about 0.1 micrometer thick, up to 3 micrometers long, and contain several small tubules (150-200 angstroms thick) embedded in a dense matrix, aligned parallel to the rod's long axis. In some cells, the endoplasmic reticulum cisternae are greatly expanded by the accumulation of a dense, finely granular material. The nature and significance of these components are unknown. In a survey of numerous sections of rat and human lungs, two types of cytoplasmic components were found in pulmonary artery endothelial cells. A subsequent survey of other organs revealed the presence of one of these components (a rod-shaped tubulated body) in practically all arterial, arteriolar, and endocardial endothelia. Similar bodies were occasionally found in the endothelia of alveolar capillaries. The purpose of this note is to call attention to these structures, although their functional significance is unknown. The lungs of rats were fixed in situ with osmium tetroxide, then removed, cut into blocks, and embedded in Epon 812. Other organs were fixed in osmium tetroxide and embedded in Epon 812. Sections were cut, mounted on carbon-coated Formvar films, contrasted with lead, and examined under a Siemens Elmiskop I. Tissues for histochemical studies were fixed in cacodylate-buffered formaldehyde or glutaraldehyde. The rod-shaped tubulated bodies are small, rod-shaped bodies of moderate density, often found in groups in the perinuclear region of the cytoplasm. Cross-sections are circular or elliptic, with a long axis up to 6-8 times the short axis. The bodies are bounded by a tightly fitting membrane (60-80 angstroms thick) and contain small cylindrical tubules (150-200 angstroms in diameter) embedded in a dense matrix. The tubules are straight and generally parallel to the rod's axis. The second type of inclusion consists of highly dilated endoplasmic reticulum cisternae filled with dense material. These cisternae are enveloped by a membrane with ribosomes and are identified as segments of the rough endoplasmic reticulum. Both components were found regularly in endothelial cells of small pulmonary artery branches in rats. Rod-shaped bodies were also found in the endothelium of alveolar capillaries, but not in dilated cisternae. The endothelium of systemic arterioles also regularly contained rod-shaped bodies. These bodies were also seen in large numbers in the atrial endocardium of ratsA previously unknown rod-shaped cytoplasmic component, consisting of a bundle of fine tubules enclosed by a tightly fitting membrane, was regularly found in endothelial cells of small arteries in rats and humans. These components are about 0.1 micrometer thick, up to 3 micrometers long, and contain several small tubules (150-200 angstroms thick) embedded in a dense matrix, aligned parallel to the rod's long axis. In some cells, the endoplasmic reticulum cisternae are greatly expanded by the accumulation of a dense, finely granular material. The nature and significance of these components are unknown. In a survey of numerous sections of rat and human lungs, two types of cytoplasmic components were found in pulmonary artery endothelial cells. A subsequent survey of other organs revealed the presence of one of these components (a rod-shaped tubulated body) in practically all arterial, arteriolar, and endocardial endothelia. Similar bodies were occasionally found in the endothelia of alveolar capillaries. The purpose of this note is to call attention to these structures, although their functional significance is unknown. The lungs of rats were fixed in situ with osmium tetroxide, then removed, cut into blocks, and embedded in Epon 812. Other organs were fixed in osmium tetroxide and embedded in Epon 812. Sections were cut, mounted on carbon-coated Formvar films, contrasted with lead, and examined under a Siemens Elmiskop I. Tissues for histochemical studies were fixed in cacodylate-buffered formaldehyde or glutaraldehyde. The rod-shaped tubulated bodies are small, rod-shaped bodies of moderate density, often found in groups in the perinuclear region of the cytoplasm. Cross-sections are circular or elliptic, with a long axis up to 6-8 times the short axis. The bodies are bounded by a tightly fitting membrane (60-80 angstroms thick) and contain small cylindrical tubules (150-200 angstroms in diameter) embedded in a dense matrix. The tubules are straight and generally parallel to the rod's axis. The second type of inclusion consists of highly dilated endoplasmic reticulum cisternae filled with dense material. These cisternae are enveloped by a membrane with ribosomes and are identified as segments of the rough endoplasmic reticulum. Both components were found regularly in endothelial cells of small pulmonary artery branches in rats. Rod-shaped bodies were also found in the endothelium of alveolar capillaries, but not in dilated cisternae. The endothelium of systemic arterioles also regularly contained rod-shaped bodies. These bodies were also seen in large numbers in the atrial endocardium of rats