We evaluated an accessory that enables the Du Pont aca discrete analyzer to measure Na+ and K+ by direct potentiometry (aca/ISE). Na+ and K+ gave linear responses in both the blood and urine modes, with no carryover. No interfering species were identified in the blood mode. Intra-assay and interassay precision for Na+ and K+ were more than adequate and analytical recoveries comparable to those with flame photometry were found for plasma. The Na+ and K+ values for plasma were comparable with those obtained by use of another direct potentiometric analyzer (the Orion SS-30), but slightly higher than flame-photometric values. Samples from patients with hyperlipemia and multiple myeloma gave clinically relevant values with the aca/ISE, but the flame photometric values were artefactually low. Whole blood could not be accurately analyzed. The aca/ISE is a precise, easy-to-use instrument, which gives values for plasma similar to those obtained with other direct potentiometric analyzers. For analysis of urine, certain precautions are necessary.