Signal transducer and activator of transcription 5A (STAT5A) has been shown to be important for terminal differentiation of mammary epithelial cells. In order to understand regulation of expression of STAT5A, the 5' end of the mouse Stat5a gene was isolated. Putative regulatory elements was searched for and several peroxisome proliferator response elements (PPREs) were found, one with high (12/13 nucleotides) and three with less (8-10/13) similarity to the reported consensus sequence. Mouse mammary epithelial HC11 cells were treated with peroxisome proliferator-activated receptor gamma (PPARgamma) ligand, the thiazolidinedione (TZD) troglitazone, and an increase in STAT5A protein expression was seen. The 5' flank of Stat5a gene was cloned in a luciferase reporter vector. A concentration dependent activation of the STAT5A-luciferase reporter was detected, when transiently transfected HC11 cells were treated with TZD. The activation could be inhibited by treatment with a PPARgamma antagonist. It has earlier been shown that epidermal growth factor (EGF) induces MAPK phosphorylation of PPARgamma resulting in a less transcriptionally active receptor. In HC11 cells, EGF inhibited TZD induced STAT5A-reporter activity suggesting that our previously reported EGF-mediated suppression of STAT5A expression is mediated in all or partly through inhibition of PPARgamma activity. Furthermore, the MEK inhibitor PD98059 inhibited the EGF effect. All together, data presented suggest that PPARgamma participates in regulation of STAT5A expression.