Several families of brain parenchyma and microvessel endothelial cell enzymes can metabolize substrates of exogenous origin. This xenobiotic metabolism includes functionalization and conjugation reactions and results in detoxication, but also possibly in the formation of pharmacologically active or neurotoxic products. The brain is partially protected from chemical insults by the physical barrier formed by the cerebral microvasculature of endothelial cells, which prevents the influx of hydrophilic molecules. These cells provide also, as a result of their drug-metabolizing enzyme activities, a metabolic barrier against penetrating lipophilic substances. The involvement of these enzymatic activities in neurotoxic events, probably responsible for neuronal dysfunctioning and/or death, neurodegenerative diseases and normal aging, is discussed.