Sodium butyrate in millimolar concentrations causes an accumulation of acetylated histone species in a variety of vertebrate cell lines. In all lines tested, butyrate caused hyperacetylation of H3 and H4, and in rat IRC8 cells, H2A and H2B were also affected. In Friend erythroleukemic cells, butyrate also induces the synthesis of a nonhistone chromosomal protein, IP25. butyrate does not affect the rate of histone acetylation in cell-free extracts of nuclei of Friend cells. Rather, this fatty acid inhibits histone deacetylation. Cell-free extracts of either control cells or butyrate-grown cells contain comparable levels of histone-deacetylating activity. This in vitro activity is inhibited by the addition of butyrate to the extracts. Thus butyrate appears to be an inhibitor of histone deacetylases both in vivo and in vitro.