The relationship between the content of sodium in the metal in industrial aluminium production cells and the current efficiency (CE) was analysed, using available theories and experimental data. The sodium concentration as well as the CE varies with the bath composition (the bath ratio) and the cathodic overvoltage. The mentioned parameters are all functions of the same type of ratio between the activities of sodium fluoride and aluminium fluoride, thus enabling an internally consistent and quantitative description. The "ideal" relationship between the sodium concentration in the metal, the CE, and the cathodic overvoltage is illustrated. It is suggested that the sodium content in the metal can indeed be used as an indicator of the cell performance, but it is necessary to use adequate sampling and analysis techniques. The exact cell conditions during sampling should also be taken into account, such as the bath composition and the overfeeding/underfeeding cycle.