Endothelium-derived relaxing factor (EDRF) and endothelium-derived contracting factor (EDCF) are released by endothelial cells following stimulation by a wide range of chemical agonists acting on specific endothelial membrane receptors and by physical factors such as shear stress on the vascular wall. It has now been firmly established that EDRF is nitric oxide, whereas EDCF has been recently identified as a 21-residue peptide, endothelin. Circumstantial evidence, however, suggests that there may be more than one EDRF and/or EDCF. EDRF induces relaxation of the underlying vascular smooth muscle by enhancing intracellular cyclic GMP levels. This action is comparable to that of nitrovasodilators, and nitric oxide can, therefore, be regarded as an endogenous nitrovasodilator. The mechanism of action of endothelin is uncertain, but depends on influx of extracellular calcium. The respective roles of EDRF and EDCF in disease are still hypothetical. It is preferable to think in terms of balance (or imbalance) between these two factors which, probably, have a fundamental role, and very likely interact with each other in maintaining and regulating vascular tone in man.