Investigation of the polar constituents of Anthocephalus cudamba by gel-permeation and ion-exchange chromatography has led to the isolation of two related indole alkaloid glycosides—cadambine (C27H32N2O10, m.p. 209'-211°, [α]D -71°) and 3a-dihydrocadambine (C27H34N2O10, [α]D25 -40°). Novel structures 1 (R = H) and 2a (R = R' = H) are proposed for them respectively based on chemical and spectroscopic evidence. Cadambine has four acylable sites, with tertiary basic nitrogen. UV spectral changes with acetic acid addition and basification suggest an equilibrium between indole/p-alkoxyacrylate chromophores and a 3,4-dihydro-β-carbolinium form. Reduction of cadambine tetra-acetate with NaBH4 in acetic acid afforded two dihydrocadambine tetra-acetates, whose 3β and 3α configurations were judged from CD spectra. 3a-Dihydrocadambine pentaacetate (2a, R = R' = Ac) was identical to the acetate derivative of the second alkaloid, linking the two structure determinations. Mass, NMR, and IR data confirmed the tetrahydro-β-carboline, methyl p-alkoxyacrylate, and glucoside moieties. Stereochemical assignments (e.g., α orientation of the alkyl residue at C-3(S) from CD) and biogenetic reasoning (e.g., 3a-dihydrocadambine derived from 18,19S-epoxystrictosidine and oxidized to cadambine) supported the structures. Cadambine and 3a-dihydrocadambine are rubenine analogues with an unusual N-4—C-18 bond in a seven-membered ring.