Extracellular glutamate can be neurotrophic or neurotoxic in the developing brain, depending on the concentration which is regulated by glutamate transporter proteins. Using immunoblotting and immunocytochemistry (ICC), we tested the hypothesis that expression of neuronal (EAAC1) and glial (GLT) glutamate transporter proteins was regionally and temporally regulated in the developing brain. Immunoblots for EAAC1 revealed a single immunoreactive (IR) band at 70kDa, whereas 73 and 140kDa proteins were seen with GLT antibodies. EAAC1 and GLT are regulated differently during development with EAAC1 being most abundant at 60 days (d) completed gestation (term=145d) and dissipating thereafter while GLT was most abundant at 136d gestation. ICC confirmed these observations and revealed that GLT is expressed by neuronal and nonneuronal cell types during mid gestation with astrocyte IR first evident at 93d and glial selectivity developing by 136d. During midgestation transient expression of GLT is seen in pyramidal cells in hippocampus, motor neurons in cranial nerve nuclei, subplate neurons, and Purkinje cells in the cerebellum. Conversely, EAAC1 expression is predominantly neuronal throughout gestation with intense labeling of dendrites within the telencephalon evident at 60d. Neuropil, neuronal cell bodies and processes are EAAC1+ throughout gestation with no evidence of astrocytic or oligodendroglial IR. The differential and developmental regulation of these proteins may have implications for our understanding of changes in cellular vulnerability to hypoxic/ischemic and traumatic insults during development. Supported by NINDS 20020, 1742-01, NLA 07914, and AHA 95009060.