Steroid/thyroid/retinoid receptors are members of the nuclear receptor superfamily and ligand-inducible transcription factors. These receptors modulate transcription of various cellular genes, either positively or negatively, by interacting with specific hormone-response elements located in the target gene promoters. Recent data show that nuclear receptors enhance or inhibit transcription by recruiting an array of coactivator and corepressor proteins to the transcription complex. We examined and compared the expression of four coactivator (steroid receptor coactivator-1 and E1A-associated 300-kDa protein) and corepressor (SMRT and N-CoR) genes in a number of tissues including several endocrine glands and cell lines. We also addressed whether their messenger RNA levels are hormonally regulated by studying the effects of thyroid hormone (T3) and estrogen (E2) treatment in rat pituitary cells (GH3) in vitro and in anterior pituitary in vivo. Our studies show that there are distinct tissue-specific expression patterns of these genes. We show that T3 and E2 regulate the expression of steroid receptor coactivator-1 messenger RNA in the anterior pituitary in addition to a gender-related difference. These tissue variations may have physiological implications for heterogeneity of hormone responses that are observed in normal and malignant tissues.