A 20-year-old white woman developed a left hemiplegia following the ingestion of LSD. Initial angiography revealed marked constriction of the internal carotid artery at the level of the siphon which, in a few days, was converted to occlusion of the carotid artery at that level. It is suggested that LSD, an amine derivative of the ergot alkaloids, was responsible for the initial vasoconstriction...