Because abnormalities of glutamatergic neurotransmission in psychiatric illness are likely not limited to glutamate receptor expression, we investigated expression of excitatory amino acid transporters (EAATs) in the striatum. The EAATs, normally expressed in both glia (EAAT1 and EAAT2) and neurons (EAAT3 and EAAT4), have previously been implicated in Huntington's disease, amyotrophic lateral sclerosis, and schizophrenia. In this study, we investigated striatal expression of transcripts encoding EAATs in tissue from mood disordered and schizophrenic subjects. With probes designed for the human EAAT1, EAAT2, EAAT3, and EAAT4 transcripts, we performed in situ hybridization and detected decreased expression of EAAT3 and EAAT4 transcripts in the striatum in bipolar disorder. We also detected decreased EAAT3 transcript expression in schizophrenia and decreased EAAT4 transcript expression in major depressive disorder. These results suggest that changes in striatal transporter mRNA expression are restricted to neuronal EAATs and extend the body of evidence implicating abnormal glutamatergic neurotransmission in schizophrenia and mood disorders. © 2002 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.