The purpose of this study was to evaluate contractility ofthe rabbit abdominal aorta 4 days after de-endothelialization by balloon catheter. The isometric tension of ring-segments in response to vasoactive agents was monitored. A significant enhancement of contraction to noradrenaline and serotonin was found in aortas 4 days after endothelium denudation as compared with controls with endothelium. The enhancement, however, did not differ from that found already in acutely denuded vessels (immediate denudation). No significant difference in contractility to potassium chloride was found in either group of denuded preparations as compared with controls. The sensitivity to all three vasoactive agents (EC<sub>50</sub>) was not influenced by denudation. These results indicate that changes in the contractility of denuded vessels are predominantly a consequence of lacking the endothelium as a producer of endothelium-derived relaxing factor. The access of mitogens to the media does not seem to interfere with the magnitude of contraction 4 days after denudation.