Endothelial cells release a relaxant factor, endothe- lum derived relaxing factor (EDRF), and mediate relaxation of vascular smooth muscle induced by various agonists. On the other hand, removal of endothelium potentiates vasoconstrictor effect of norepinephrine (NE). In the present study, it is shown that removal of the endothelium of isolated rabbit thoracic aortic strips, causes a significant increase in the slopes of the concentration-response curves to NE obtained repetitively after the control curve. An earlier desensitization was observed in rubbed preparations when compared with unrubbed strips. These findings suggest that removal of endothelium alters the characteristics of the concentration-response curves to NE and this may cause wrong interpretations if endothelium was not kept intact.