Isotonic contractions and transmembrane resting potentials were recorded both before and after the addition of ethylenediaminetetraacetic acid (EDTA) (4 mm final conc.) to toad hearts perfused with Ringer's solution containing either Ca++ or Sr++. After EDTA perfusion the introduction of Ringer's solution containing Sr++ resulted in contracture and a low resting potential (30 – 48 mv); the introduction of Ringer's which contained Ca++ resulted in restoration of normal contractility and transmembrane potentials. These results are discussed according to the hypothesis that Sr++, unlike Ca++, does not maintain the resting membrane potential.