in GH-induced activation of many genes including IGF-I, Socs2, Cis, IGFALS, and Spi2.1. Among these genes, unique features of IGF-I include that its transcribed region of >70kb is substantially larger than the others, that it uses two independent promoters to direct transcription, and that its promoters lack Stat5 binding domains, which are instead scattered throughout its gene locus. We have recently characterized chromatin events at the two IGF-I promoters following acute GH stimulation in a hypophysectomized rat model, and observed that the mechanisms of activation of the two promoters are distinct. Whereas RNA polymerase II is recruited to promoter 2 upon GH treatment, both RNA polymerase II and the co-activator p300 occupy promoter 1 even in the absence of GH, indicative of a poised state. Here we sought to characterize the acute epigenetic actions of GH at a cohort of hormone-stimulated, Stat5b-dependent genes. We first found that the transcription of all five genes tested was rapidly stimulated by GH treatment, but as measured by quantitative transcription assay, the magnitude of gene activation for IGF-I, Socs2, and Cis was over 10-times greater than for ALS and Spi2.1. Histone modifications at the promoters were similar in the genes with acute GH-stimulated increases is H3 and H4 acetylation and significant enrichment in H3 K4 trimethylation, with the exception of Spi2.1 which did not possess these marks typical of activated genes. As expected, Stat5b was recruited to the promoters of the set of genes in a GH-dependent way, but interestingly we found the transcriptional coactivators p300 and TRAP220/Med1 were present at IGF-I promoter 1 but not at any of these four genes. RNA polymerase II was detected at transcribed segments following GH in all the genes, but was most abundant upstream of the transcription start site in the basal state at IGF-I promoter 1. Finally we detected enrichment of the transcriptional repressor Bcl6 at the promoters of the Socs2 and Cis genes in the absence of hormone signaling, with acute loss of this signal upon GH treatment coincident with the recruitment of Stat5 to the same regions. In contrast, the levels of Bcl6 before and after GH at IGF-I promoter 1 were similar to an unrelated control segment. In summary, we conclude that gene activation by GH via Stat5b involves distinct promoter-specific mechanisms utilizing different transcriptional co-regulatory complexes.