Lethal doses (pigeon) were measured of the new cardiotonic glycosides reported to date in papers of this series, i.e. gitostin, neogitostin, and glucodigifucoside, the known glycosides, i.e. digitalinum verum, digitoxin, strospeside, and purpurea glycoside-B, and acetyl derivatives such as 16-acetyl-digitalinum verum derived chemically from digitalinum verum, and relationship between their chemical structure and toxicity was comparatively examined. An interesting fact was thereby found that 16-acetyl-digitalinum verum was about five times stronger than digitalinum verum.