We present evidence that both corepressors SMRT and N‐CoR exist in large protein complexes with estimated sizes of 1.5–2 MDa in HeLa nuclear extracts. Using a combination of conventional and immunoaffinity chromatography, we have successfully isolated a SMRT complex and identified histone deacetylase 3 (HDAC3) and transducin (β)‐like I (TBL1), a WD‐40 repeat‐containing protein, as the subunits of the purified SMRT complex. We show that the HDAC3‐containing SMRT and N‐CoR complexes can bind to unliganded thyroid hormone receptors (TRs) in vitro. We demonstrate further that in Xenopus oocytes, both SMRT and N‐CoR also associate with HDAC3 in large protein complexes and that injection of antibodies against HDAC3 or SMRT/N‐CoR led to a partial relief of repression by unliganded TR/RXR. These findings thus establish both SMRT and N‐CoR complexes as bona fide HDAC‐containing complexes and shed new light on the molecular pathways by which N‐CoR and SMRT function in transcriptional repression.