Elongation factor-1 alpha (EF-1 alpha) is a highly conserved, abundantly expressed protein that functions in peptide elongation during mRNA translation. Mammalian species contain a second EF-1 alpha gene, S1, whose expression is limited to brain, heart, and skeletal muscle. Such tissue specificity in S1 gene expression led us to hypothesize that this specialized member of the EF-1 alpha family is possibly essential to terminally differentiated, long-lived cells such as neurons, cardiomyocytes, and myocytes. We report here that during development, EF-1 alpha mRNA levels remain unchanged in S1-negative tissues, but sharply decrease in S1-positive tissues. RNase protection as well as tissue distribution analyses of the S1 message suggest that down-regulation of EF-1 alpha expression coincides well with that of S1 up-regulation during postnatal development. Analysis of primary cultures of the rat neonatal cerebral cortex cells has shown that S1 gene expression is indeed restricted to neurons only, whereas nonneuronal cell types such as astrocytes and microglia are S1-negative. EF-1 alpha mRNA on the other hand was detected in all three cell types, namely, neurons, astrocytes, and microglia. This report confirms the terminal differentiation-dependent expression of the S1 gene in neurons of the cerebral cortex.