The 13C n.m.r. data of the Digitalis and the Cerbera cardiac glycosides, and of ouabain and neriifolin, have been analysed. Both 13C and 1H n.m.r. results show that the 11,12-epoxide in cerbertin has an α-configuration. Here we present the 13C chemical-shift data of a number of natural cardenolides and their derivatives, viz. the Digitalis tri-β-D-digitoxosides digitoxin (1), digoxin (2), and 3'''-acetyldigoxin (3); the α-L-thevetosides neriifolin (6), cerbertin (8), 4'-acetylcerbertin (9), and deacetylcerbertin (7); and the α-L-rhamnoside ouabain (10). For comparison, data of the genins digitoxigenin (4) and digoxigenin (5) are also included.