A subset of transcription factors function as pivotal regulators of cell differentiation pathways. Pituitary transcription factor -1 (Pit-1) is a tissue-specific homeodomain protein that specifies the development of pituitary somatotropes and lactotropes. In this study, adenovirus was used to deliver rat Pit-1 to mouse liver. Pit-1 expression was detected in the majority (50-80%) of hepatocyte nuclei after tail vein injection (2 x 10⁹ plaque forming units). Pit-1 activated hepatic expression of the endogenous prolactin (PRL), GH (GH), and thyroid stimulating hormone (thyrotropin) genes along with several other markers of lactotrope progenitor cells. Focal clusters (0.2-0.5% of liver cells per tissue section) of periportal cells were positive for PRL by immunofluorescent staining. The PRL-producing cells also expressed proliferating cell nuclear antigen (PCNA) as well as the hepatic stem cell markers (c-kit, thy1, and cytokeratin 14). These data indicate that Pit-1 induces the transient differentiation of hepatic progenitor cells into PRL-producing cells, providing further evidence that transcription factors can specify the differentiation pathway of adult stem cells.