In this work, we test the idea that most, if not all, cellular Cl- of Amphiuma red blood cells is contained in the cytoplasm. If true, this could resolve the difference between the measured plasma membrane potential (Em) and that expected from the Donnan equilibrium distribution of Cl-. We studied the changes in the fluorescence intensity of the Cl- -sensitive dye, MQAE, entrapped in red cells that occurred when intracellular Cl- was exchanged with NO3-. We could thus monitor the distribution of Cl- between the nuclear and cytoplasmic compartments. We found that essentially all of the cell's Cl- resides in the cytoplasm. Knowing the volume of the cell occupied by the nucleus, we could accordingly correct the measured values of cell Cl-. This resulted in establishing a concordance between the measured values of Em and those calculated from the corrected values of the Cl- ratio, thus explaining the discrepancy. The exclusion of Cl- from the nucleus may result from its unusually high content of "excess" DNA that imposes an imbalance of net negative charge.