['The endothelium-derived relaxing factor that mediates the actions of acetylcholine is now most frequently identified as nitric oxide. Nitric oxide is believed to have numerous important regulating actions in neurons, blood vessels, and several other biological systems.', 'The literature concerning tissue other than cerebral blood vessels supports the conclusion that the endothelium-derived relaxing factor for acetylcholine is either nitric oxide or a compound formed from and containing nitric oxide (for example, a nitrosothiol). However, papers can be found indicating that this endothelium-derived mediator is not nitric oxide. In brain blood vessels the evidence is strongly against the conclusion that nitric oxide is the endothelium-derived mediator for acetylcholine. If this mediator is formed from nitric oxide, either in brain vessels or in other vessels, no data are available delineating how this synthesis is regulated or whether and where nitric oxide leaves the nitroso compound to initiate dilation. Indeed, cerebrovascular data now cast doubt on the commonly held belief that nitrosovasodilators regulate vascular tone by giving off nitric oxide to vascular smooth muscle.'] Is endothelium-derived relaxing factor in brain blood vessels nitric oxide?