Corticosterone significantly increases the incorporation of [3H]leucine into specific cytosol protein(s) isolated from in vitro hippocampal slices prepared from adult male albino rats. The present study showed that in slices coincubated with glucocorticoid plus a protein synthesis inhibitor (1 mM-cycloheximide), no such enhancement of amino acid incorporation was observed, suggesting that the hormone acts in the hippocampus to increase de novo protein synthesis. Further experiments demonstrated that the steroid-induced protein synthesis was first detectable (+ 5.7%) following a 30-min exposure of slices to corticosterone; slices incubated for 1 or 2 h both showed a 12% increase in synthesis of the affected protein(s) when compared with controls. In an attempt to determine whether the glucocorticoid alteration of protein metabolism was receptor-mediated, hippocampal slices were also incubated with 10 nM-progesterone, a steroid known to compete for corticosterone binding to its cytosol receptor. Progesterone alone, which does not translocate cytoplasmic receptors to the nucleus, did not alter hippocampal protein metabolism and effectively blocked the induction by corticosterone of the 54K protein(s). These studies provide evidence that in the rat hippocampus corticosterone interacts with high-affinity steroid receptors to regulate the synthesis of specific protein(s).