This Thesis is divided into three parts. In Part I the electrical activity and intracellular ionic contents of sodium, potassium and chloride in the rat auricle have been investigated, to test a hypothesis advanced by Shanes (l958) to account for the action of 2-4-dinitrophenol on the heart. In Part II some observations on arrhythmias produced by 2- 4- dinitrophenol in the heart have been extended and investigated. In Part III the results of a few polarisation experiments are presented. Since the main body of the work has been done and the Thesis written, additional experiments on the efflux of chloride using Br⁸² as .tracer have been carried out. The results of tnese experiment have been added in the results of Part I.