In the present paper 18 naturally occurring monosides and triosides and some semi-synthetic glycoside derivatives were investigated on their positive inotropic and arrhythmogenic properties in the isolated spontaneously beating guinea-pig heart. It could be shown that: (a) substitution of C19-OH causes a considerable decrease in efficacy; (b) acetylation of 16α-OH or 16β-OH leads to a 6–7-fold increase in efficacy; (c) esterification of hydroxy groups of the sugar moiety brings about only a slight decrease in efficacy regarding the range of inotropic effective concentrations, but decreases the maximal inotropic effect, and (d) that a dissociation between inotropic and arrhythmogenic effects is observed especially in the case of 16-acetyl·16-epi-gitoxin. The present findings are compared with former results obtained in the auricle preparation and with I50-values of the membrane ATPase system.