THE positive inotropic effect of digitalis compounds is generally attributed to an increase in the amount of activator calcium¹ and not to a change in the calcium sensitivity of regulatory contractile proteins^2,3. The main question is the cellular or subcellular basis of the increase in the myoplasmic calcium transient. Since calcium entry through the slow inward current may play an important part in excitation–contraction coupling, it has been suggested that the slow inward current might be enhanced by digitalis^4,5. All the evidence has been negative^6–11 apart from a report by Dramane et al.¹² and results of H. Brown, W. Giles and S. Noble (personal communication). We have re-examined this question and have found that strophanthidin can increase the slow inward current and alter its re-priming kinetics in certain conditions. These observations may provide a partial explanation of the positive inotropic effect observed in the same experiments.