The research of A. Szent-Györgyi and co-workers1on the biochemistry of muscular contraction, while not universally accepted with respect to certain details of energy transfer, has provided a useful mechanochemical model of the events occurring in the contraction of the myocardium. The purpose of this paper is an attempt to apply some of the mechanical aspects of this hypothesis in a definitive manner to acute digitalis intoxication and to avoid for purposes of simplification the controversial issues involved in consideration of the effect of digitalis on the important energy-yielding and energy-utilizing biochemical mechanisms. At the cellular level, Hajdu and Szent-Györgyi2assign a regulatory function to digitalis with respect to the creation of the proper ionic environment for optimal contraction of the actomyosin protein complex in heart muscle. The experimental basis for this concept is the study of the isometric tension developed by the isolated frog ventricle under a