Arterial smooth muscles behave like a syncytium, since they are electrically coupled. It is generally assumed that electrical coupling and dye coupling are mediated by gap junctions. No gap junctions could be detected by transmission electron microscopy in media of coronary arteries. We looked for the presence of gap junction protein in vascular smooth muscle by immunohistochemistry with light microscopy. Immunohistologically detectable connexin is expressed by smooth muscle cells of the media of pig coronary arteries, where staining occurs as a discrete punctation. We investigated the dye coupling in strips of pig coronary artery. The fluorescent dye lucifer yellow was microiontophoretically injected into a smooth muscle cell through an intracellular microelectrode. The dye was visualized on the entire strip, then on semithin sections with a fluorescence microscope, and at the ultrastructural level by using an anti-lucifer yellow antibody revealed by the protein A-gold technique. In all the tissues examined, the cells were dye-coupled. We conclude that in arterial media the smooth muscle cells are dye-coupled, despite the absence of detectable gap junctions by transmission electron microscopy, and suggest that dye coupling could occur via isolated gap junction channels.