We present a comparative evaluation of the electrode systems of three modern blood gas analysers: IL-413, ABL-1 and AVL-937C. The response curves, accuracy and precision of the pH-, pCO2- and pO2-electrodes were established with tonometered blood and buffer solutions. pH values (range 6.8-7.8) measured on the AVL deviate (-0.03 pH for blood and +0.03 pH for buffer) from those of BMS2 Mk2; whereas on the IL and ABL analysers the pH values deviate by not more than 0.01 pH. The standard deviation was better than 0.005 pH. pCO2 values of blood and buffer (range 14-106 mm Hg) deviate from the calculated tonometer values by quantities ranging from 3 to 10 mm Hg. The average precision (CV)1) of the pCO2 measurement on each analyser was better than 1.8%. pO2 values of blood (range 0-130 mm Hg) did not differ by more than 3 mm Hg from the calculated values. Above 130 mm Hg a linear negative increasing difference was seen. For buffer solutions a linear relationship between pO2 difference and pO2 value was found over the whole range from zero up to 642 mm Hg: a positive difference below and a negative difference above the pO2 of the previous calibration; if the calibration pO2 is higher, the sample pO2 is shifted to a higher value. The average precision of the pO2 measurements was better than 3%. In the (patho)-physiological range the three instruments may provide suitable results for the clinician. Suggestions are made for standardization and improvement of the electrode systems.