The selectivity of a glass capillary-based L-glutamate sensor toward protein, inhibitors, amino acids and some compounds related to a neurophysiological study were investigated in an artificial cerebrospinal fluid (ACSF) and Ca²⁺, Mg²⁺–free ACSF. The response of the capillary sensor at 0 mV vs. Ag–AgCl was not affected by implantation into brain slices, showing that the adsorption of proteins is negligible. Most of inhibitors, such as D-APV and DNQX, ifenprodil and (−)bicuculline, which are often used in electrophysiological studies, did not affect the response of the sensor, as long as their concentrations are physiologically used levels. However, DL-TBOA, THA and DMSO affected the sensor response, seemingly by blocking the capillary action of the sensor. L-Ascorbic acid (<200 μM), L-aspartic acid (<200 μM) and glutamine (<100 μM) did not interfere with the response. The selectivity data are useful for the application of the sensor for monitoring L-glutamate in brain.