Oscillatory contractions in uterine smooth muscle are mechanistically related to gap junction complex formation. We have tested the hypothesis that agonist-induced oscillations in vascular smooth muscle are also mediated by gap junctions and that gap junctions are important for vascular smooth muscle cell communication. Total RNA from cultured Wistar-Kyoto rat (WKY) mesenteric arterial cells hybridized strongly with a cDNA probe for the message for connexin43, a monomer of the gap junction. In these same cells, the quaternary ion tetraethylammonium (TEA) (10 m M) increased Lucifer yellow dye transfer between contiguous cells, a measure of cell-to-cell communication via gap junctions, approximately 35% above basal levels. Heptanol, an established inhibitor of gap junction communication, completely blocked both basal- and TEA-stimulated dye transfer between neighboring cells. In other experiments, helical strips of superior mesenteric and tail arteries from WKY rats were mounted in tissue baths for measurement of isometric contractile force. TEA (10<sup>–3</sup>–10<sup>–1</sup> M) induced oscillatory contractions (1–5 cycle/min) in both mesenteric and tail arteries. Removal of endothelium did not affect the pattern of TEA-stimulated oscillations. Oscillations to TEA were blocked in a concentration-dependent manner in both arteries by heptanol (10<sup>–7</sup>–10<sup>–3</sup> M). Heptanol (10<sup>–3</sup> M) also significantly reduced (40%) acetylcholine-induced relaxation in the mesenteric artery (contracted with phenylephrine). Thus, these data document that: (1) the message for the gap junction protein connexin43 is located in cultured arterial cells; (2) TEA induces oscillatory contractions in vascular smooth muscle and stimulates intercellular communication in cultured cells, and (3) oscillatory contractions, vascular relaxation and intercellular communication are blocked by the gap junction inhibitor heptanol. Collectively, these data strongly support the importance of gap junctional communication in vascular smooth muscle reactivity, including both oscillatory contractions and vascular relaxation.