The influence of cervical and periarterial sympathectomy on endothelium-dependent relaxation of the rabbit common carotid artery was studied in vitro. Noradrenaline, serotonin, acetylcholine and papaverine concentration-response curves of sympathectomized and control rabbit common carotid artery rings were recorded and analyzed and effects of endothelium removal were investigated. Endothelium-dependent relaxation was selectively and significantly inhibited in 3 weeks postsympathectomy arterial preparations. Alterations in acetylcholine receptors of endothelial cells, decrease in endothelium-derived relaxing factor synthesis, or desensitization of medial muscular cells to endothelium-derived relaxing factor are hypothetical causes of this phenomenon.