We investigated the alkaloids of the epigeal part of Arundo donax (giant reed) from the "Subtropik" sovkhoz in Shaartuz region, Tadzhik SSR. From 60 kg of plant material, 0.18% ether-soluble and 0.04% chloroform-soluble bases were obtained via chloroform extraction. Donaxine (I) was isolated from ether-soluble bases by acetone treatment, and column chromatography of the mother liquor on Al₂O₃ yielded new alkaloid donaxaridine (II, mp 175-176°C, [α]ᴅ ± 0°, C₁₁H₁₄N₂O₂) and known donaxarine (III, mp 218-220°C, [α]ᴅ ± 0°, C₁₃H₁₆N₂O₂). Spectroscopic (IR, UV, NMR, mass) and chemical analyses (acetylation, oxidation, condensation) revealed donaxaridine as 3-(β-methylaminoethyl)dihydroxyindole and donaxarine as oxindole-3-spiro-6'-(2',3'-dimethyltetrahydro-1,3-oxazine). Additionally, we re-evaluated l-cycloprotobuxine-C (I, mp 195-197°C, [α]ᴅ -62°, C₂₇H₄₃N₂), previously thought to be the levorotatory form of cycloprotobuxine-C (II). Comparison with an authentic sample showed physicochemical differences, indicating it is a stereoisomer. Mass spectrometry, NMR (methyl proton shifts), and saponification rate studies suggested the C₂₀ N(CH₃)₂ has α orientation and C₃ NH-CH₃ has e-axial orientation, leading to the structure 20α-dimethylamino-4,4,14a-trimethyl-3α-methylamino-9α,19-cyclo-5α-pregnane.