The phytochemical investigation of alkaloids in the flowers of Arundo donax was conducted as no previous work on its flowers had been reported. Two-week-old flower material was defatted and extracted with hot ethanol, followed by solvent extraction at different pH levels and isolation via reineckates. Isolated alkaloids included gramine, N,N-dimethyltryptamine, bufotenine, 5-methoxy-N-methyltryptamine, a new compound eleagnine (the first tetrahydro-β-carboline alkaloid from a graminaceous plant), 3,3'-bisindolylmethyl dimethylammonium hydroxide, gramine methohydroxide, and N,N-dimethyltryptamine methohydroxide. Additionally, gramine content in the flowers remained constant for the first two weeks post-appearance, then declined and was replaced by quaternary amine and bis-compound, suggesting that quaternary gramine compound could be a plausible precursor of gramine metabolites in higher plants.