Extracellular tissue spaces are compared using inulin, sucrose, chloride and sodium in hearts perfused with Ringer-Locke solution by Langendorff's method. Also chloride and sodium spaces are given for unperfused, fresh hearts. From the inulin spaces the intracellular chloride, sodium, potassium and water concentrations in both perfused and unperfused heart chambers have been calculated. Data showing the specificity of the atrium and ventricles of the heart are presented.