Microsomal glutathione S-transferase type 3 (MGST3) is a recently identified member of a large superfamily of enzymes involved in biotransformation of xenobiotics and biosynthesis of eicosanoids, including prostaglandins and leukotrienes. Using in situ hybridization histochemistry and reverse transcription polymerase chain reaction, we characterized the expression of MGST3 mRNA in the rat nervous system based on the cloned rat MGST3 gene, under normal conditions and after systemic administration of lipopolysaccharide (LPS). The MGST3 mRNA seemed to be confined to neurons. The broad distribution in the brain was characterized by a strong signal in the hippocampal formation and in the nuclei of the cranial nerves. A moderate signal was found in the cortex, thalamus, amygdala and substantia nigra and a weak signal in the hypothalamus. Motoneurons in the spinal cord and sensory neurons in dorsal root ganglia displayed strong MGST3 mRNA signal. No significant changes in the level of expression of MGST3 mRNA in the brain were found 1, 3 or 6 h after LPS administration. The pattern of distribution of MGST3 mRNA in the rat nervous system and the lack of response to LPS do not support a role for MGST3 in the biosynthesis of proinflammatory eicosanoids but rather suggest other functions, perhaps in metabolic detoxication and neuroprotection.