The endothelium mediates a number of responses (relaxation or contraction) of arteries and veins from animals and humans. The endothelium-dependent relaxations are due to the release, by endothelial cells, of potent non-prostanoid vasodilator substances. Among these, the best characterized is endothelium-derived relaxing factor (EDRF), which is believed to be nitric oxide (NO). Nitric oxide is formed by the metabolism of L-arginine by the constitutive NO synthase of endothelial cells. In arterial smooth muscle, the relaxation evoked by EDRF is explained by the stimulation by NO of soluble guanylate cyclase that leads to the accumulation of cGMP. In a number of animal blood vessels and in human coronary arteries, the endothelial cells release a substance that causes hyperpolarization of the cell membrane (endothelium-derived hyperpolarizing factor, EDHF). The release of EDRF from the endothelium can be mediated by both pertussis toxin-sensitive (a2-adrenoceptor activation, serotonin, aggregating platelets, leukotrienes) and insensitive (adenosine diphosphate (ADP), bradykinin) G proteins. In blood vessels from animals with regenerated and reperfused endothelium, and/or atherosclerosis, there is a selective loss of the pertussin toxin-sensitive mechanism of EDRF release, which favours the occurrence of vasospasm, thrombosis and cellular growth. The available information from isolated human blood vessels or obtained in situ concurs with the conclusions reached from studies with isolated animal tissues. In addition to relaxing factors, the endothelial cells can produce contracting factors (endothelium-derived contracting factors; EDCFs) which include superoxide anions, endoperoxides, thromboxane A2 and endothelin. From animal studies it can be concluded that the propensity to release EDCFs is maintained, or even augmented, in diseased blood vessels. The switch from a normally predominant release of EDRFs to that of EDCFs may play a crucial role in atherosclerosis.