A method is described for the measurement of gamma-amino-4- butyric acid (GABA) within a microdialysis probe using an antibody- linked assay. A monoclonal antibody for GABA provides the specificity of measurement and these antibodies are affixed on a working platinum electrode within the probe. Determination of bound GABA is performed via an indirect assessment of competitive ligand also bound, and conjugated to horseradish peroxidase which is activated and measured as current change. Using this probe directly for extracellular measurements in the somatosensory cortex of sheep compared favorably to the use of this probe on dialysate emerging from a classical microdialysis probe suggesting that it could be used directly in vivo. The probe has a fast response time (>90% of maximum response within 30 s of start of analysis) and high sensitivity (< 0.5 μmol/l) and can acquire data every 2 min. It is stable over a period of time in vivo (>48 h) and is regenerable. The probe design offers on-line measurement, with rapid time resolution of substances not amenable to enzyme-based amperometric measurement.