A method has been described by which triple-barrelled ion-sensitive microelectrodes for simultaneous measurements of two extracellular ion activities could be produced with a tip diameter of 1-2 microns. The following combinations with two ion exchangers could be achieved: Ca++/K+, Ca++/Cl-, K+/Na+. The mean steepness for the potassium electrode was 55.8 mV, for the calcium electrode 26 mV, for the chloride electrode 48.8 mV, and for the sodium electrode 55.2 mV. The change of steepness varied 4%-5%/day and some of the electrodes could be used up to 5 days. The mean response time for all electrodes did not amount to more than 200 ms. An example is given for simultaneous in vivo measurement of both extracellular potassium and calcium activity.
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