The aim of this project was to characterise a first-generation polymer enzyme composite (PEC) biosensor utilising lactate oxidase for the real-time neurochemical monitoring of lactate. Lactate, which was originally thought to be nothing but a mere waste product of anaerobic respiration, has gained recognition as a cell signalling molecule and playing a significant role in long term potentiation. It has been postulated that lactate may act as a primary energy substrate following glutamatergic transmission and this idea is foundation of the astrocyte lactate neuron shuttle hypothesis (ALNSH) which was proposed by Pellerin and Magistretti in 1994. This theory states that following glutamatergic transmission lactate is exported from astrocytes to neurons where it is used for oxidative phosphorylation. Lactate has also shown promise as a biomarker for sleep and lactate dysregulation in the brain has been shown to be a feature of schizophrenia and autism spectrum disorder. Electrochemical PEC biosensors used for the monitoring of other neuro metabolites such as glucose and pyruvate have been previously developed and characterised by members of the Lowry research group. A PEC lactate biosensor has also been previously developed, so the core components (cross-linking/stabilising agents and enzyme), layering strategies and drying times have been optimised to provide a sensitive sensor that now required to be characterised in vitro and in vivo. Tests included verifying the sensors signal over the range of temperatures and pHs it would encounter. As the brain is a harsh environment with low [O2] compared to benchtop conditions, oxygen dependence tests were performed to ensure the signal given on benchtop conditions was representative of what will be seen at vivo oxygen concentrations. Biocompatibility tests were performed to ensure minimal signal loss from electrode poisons, surface modifying agents and surfactants. Finally, interference testing was done to ensure minimal signal contribution from electroactive reducing agents, electrocatalysts, and endogenous electroactive interferents e.g. ascorbic acid. In summary, it showed appropriate sensitivity (0.242 ± 0.014 nA/µM) and excellent stability/biocompatibility with no significant decrease in sensitivity following ex-vivo storage in rodent brain tissue (14 days) or 28 days in dry storage. Interference from endogenous electroactive interferents was minimal with extracellular concentrations typically being ~3% of the 350 µM L-lactate response. Changes in molecular oxygen (the natural enzyme mediator) over the normal range found in brain tissue (40-80 µM) had minimal effect on the L-lactate signal for concentrations of 350 and 500 µM (KMO2 of 39.4 ± 17.5 µM and 40.4 ± 16.1 µM respectively). Alongside this a low µM limited of detection (0.153 ± iv 0.098), was calculated with a slower than expected response time of ca. 18 seconds, combined with no effect of pH and temperature changes over physiologically relevant ranges (7.2-7,6 and 34-40 °C respectively), collectively suggest this composite biosensor has the ability to reliably monitor L-lactate in the brain. The sensor was then implanted into the striatum of freely moving rats where it demonstrated stable recording over several weeks and reliable detection of physiological changes in L-lactate. The sensors’ ability to provide a reliable signal with changes in [O2] was proven, following this changes in L-lactate were monitored in response to neuronal activation (tail-pinch). Future work will focus on pharmacological interventions and the effect these have on extracellular fluid levels of lactate, which will hopefully aide in our understanding of how these interventions affect brain metabolism.