The expression of 10 histone deacetylases (HDAC1-10) mRNAs in mouse neuroblastoma Neuro-2a and microglia N9 cell cultures after treatment by inhibitors of HDACs, sodium butyrate and trichostatin A was studied to elucidate whether HDAC inhibitors affect the gene expression of HDACs themselves. Northern blot analysis demonstrated two- to four-fold elevated levels of mRNAs for HDAC1, -3, -5, and -6 after drug treatment in comparison with untreated cells, while mRNA levels for HDAC2 and -7 did not change significantly. In both Neuro-2a and N9 cells the highest increase was observed for HDAC5 and -6 mRNA, whereas, HDAC4 had a prominent increase in mRNA levels after drug treatment only in N9 microglia cell line but not in Neuro-2a. Immunocytochemical examination confirmed that changes in protein levels of HDACs were similar to changes in their mRNA levels. There exists an auto-regulatory feedback loop to the expression levels of several HDACs after inhibition of their biochemical activity, adding several HDAC genes to the list of genes regulated by HDAC inhibitors.