Nitric oxide (NO) is an important free radical synthesized and released by brain cells. At low (nanomolar) levels, it modulates synaptic transmission and neuronal activity, but at much higher levels mediates neuronal injury through oxidative stress. However, the precise concentrations at which these biological actions are exerted are still poorly defined. Electrochemical detection of NO in vivo requires rigorous exclusion of endogenous redox molecules such as ascorbate or nitrite. A fluorinated xerogel composed of trimethoxymethylsilane and heptadecafluoro-1,1,2,2-tetrahydrodecyl silane has been proposed to create a screening layer around NO sensors, protecting against such chemical interference in vitro. Here we detected NO in the living brain using carbon fiber microelectrodes covered with nickel porphyrin and this fluorinated xerogel. These microsensors were insensitive to interfering redox molecules and surpassed similar microelectrodes coated with a Nafion screening layer. In vivo, in the rat parietal cortex, these electrodes could detect brain NO released by local microinjection of the glutamatergic agonist N-methyl-d-aspartate (NMDA). NMDA-evoked NO release peaked at 1.1 μM and lasted more than 20 min. This fluorinated xerogel screening layer can therefore be applied in vivo, allowing for the fabrication of highly specific microsensors to study NO physio-pathological actions in the brain.