✓ In vivo experiments in dogs demonstrated that physiological concentrations of serotonin, when injected intracisternally, caused cerebral arterial spasm that lasted for at least 3 hours. Comparable spasm was produced by the injection of blood containing approximately the same amount of serotonin. Phenoxybenzamine reversed both the spasm produced by pure serotonin and that produced by blood. A hypothesis of the etiology of cerebral arterial spasm is proposed based on the experimental results of the entire study.