I compared results for sodium and potassium in whole blood and plasma as measured with a newly available potentiometric analyzer, the 'Orion SS-30'. No significant difference was found for either sodium or potassium in 207 such comparisons. With use of the flowing, high mixing-velocity liquid junction of the Orion SS-30, the residual liquid junction potential due to blood cells was found to be less than 0.1 mV and to be independent of the hematocrit. This is in contrast to the hematocrit-dependent residual liquid junction potential of ∞0.6 mV noted by others at normal hematocrit values with the open capillary liquid junctions now commonly used in pH instruments. I also found that the potassium concentration can increase significantly during the mixing of whole blood, and such samples should be mixed gently, if at all. Evidently sodium and potassium can be accurately and easily measured directly in heparinized blood.