1. Chloride liquid ion exchanger micro-electrodes have been used to measure intracellular chloride activity (aiCl) in isolated pieces of frog sinus venosus, atrium and ventricle. 2. aiCl was found to be 35-0 +/- 3-2, 17-3 +/- 0-8 and 17-6 +/- 0-8 mM in sinus venosus, atrium and ventricle, respectively. 3. In atrium and ventricle aiCl is independent of whether the tissue is beating or non-beating. 4. In non-beating ventricle aiCl changes rapidly and reversibly when aOCl is reduced by replacing chloride with glucuronate. 5. ECl calculated from the measured aiCl has been compared with ECl calculated from the constant field equation assuming either constant chloride conductance or constant chloride permeability. 6. It is suggested that chloride is actively transported into the cells in all three kinds of frog heart tissue.