The Ikaros transcription factors perform critical functions in the control of lymphohematopoiesis and immune regulation. Family members contain multiple zinc fingers that mediate DNA binding but have also been implicated as part of a complex chromatin-remodeling network. We show here that Ikaros is expressed in pituitary mammosomatotrophs where it regulates the GH and prolactin (PRL) genes. Ikaros was detected by Northern and Western blotting in GH4 pituitary mammosomatotroph cells. Wild-type Ikaros (Ik1) inhibits GH mRNA and protein expression but stimulates PRL mRNA and protein levels. Ikaros does not bind directly to the proximal GH promoter but abrogates the effect of the histone deacetylation inhibitor trichostatin A on this region. Ikaros selectively deacetylates histone 3 residues on the proximal transfected or endogenous GH promoter and limits access of the Pit1 activator. In contrast, Ikaros acetylates histone 3 on the proximal PRL promoter and facilitates Pit1 binding to this region in the same cells. These data provide evidence for Ikaros-mediated histone acetylation and chromatin remodeling in the selective regulation of pituitary GH and PRL hormone gene expression.