Evaluation of an apparatus for the direct measurement of sodium and potassium by means of ion-selective electrodes yielded very good results. This apparatus is notable for excellent linearity. It possesses a low detection threshold, the lowest identification limit being 4.43 mmol./l. for sodium and 0.21 mmol./l. for potassium. The sensitivity is high, since it is approximately 1.5 mmol./l. for sodium in whole blood and approximately 0.15 mmol./l. for potassium. On checking the accuracy of the instrument, a mean recovery of 100% was seen for both electrolytes. An examination of the precision of the instrument yielded, in serial determinations of sodium, variation coefficients between 0.2 and 0.4%, and in case of potassium, variation coefficients around 1%; even in respect of day-to-day precision, all variation coefficients were definitely below 5%. Hence, the overall precision of the apparatus can be rated as good. However, when using whole blood samples anticoagulated with sodium heparinate solution, it is necessary to take the dilution effect into account when arriving at the potassium value.