Experiments were designed to determine the contribution of endothelial cells to the heterogeneous behavior of the arterial and venous wall. Rings of canine femoral, pulmonary, saphenous, and splenic arteries and veins, with and without endothelium, were mounted for isometric tension recording in Krebs-Ringer bicarbonate solution. Endothelium-dependent inhibitory responses to acetylcholine, adenosine triphosphate, bovine thrombin, and arachidonic acid were prominent in the arteries. In the veins, only transient endothelium-dependent relaxations to these substances were observed. Removal of the endothelium decreased the augmentation of the response to norepinephrine caused by anoxia in both arteries and veins. In the veins, arachidonic acid and thrombin caused endothelium-dependent increases in tension during contractions evoked by norepinephrine. The endothelium-independent inhibitory effects of isoproterenol and adenosine and the excitatory effects of acetylcholine and ATP were more pronounced in the veins than in the arteries. These experiments demonstrate that in the arterial and venous wall the endothelial cells can contribute to both inhibitory and excitatory responses of the smooth muscle cells of the media. Inhibitory endothelial responses prevail in the arteries, and excitatory ones in the veins.