Dynamic acetylation and deacetylation of nuclear histones is essential for regulating the access of chromosomal DNA to transcriptional machinery. The source of acetyl-CoA for histone acetylation in mammalian cell nuclei is not clearly known. We show that acetylcarnitine formed in mitochondria, is transported into cytosol by carnitine/acylcarnitine translocase, and then enters nucleus, where it is converted to acetyl-CoA by a nuclear carnitine acetyltransferase and becomes a source of acetyl groups for histone acetylation. Genetic deficiency of the translocase markedly reduced the mitochondrial acetylcarnitine dependent nuclear histone acetylation, indicating the significance of the carnitine-dependent mitochondrial acetyl group contribution to histone acetylation.