Pituitary homeobox 1 (Ptx1) is a homeodomain-containing transcription factor acting on transcription of all pituitary hormone genes. Its expression is first detected in the stomodeal ectoderm and is maintained in all derivatives of this structure, including Rathke's pouch. We now show that Ptx1 is expressed in all pituitary cells but that it is differentially expressed in different lineages at both the messenger RNA and protein levels. On day 12.5 of mouse embryonic development, cells expressing the highest levels of Ptx1 are restricted to the forming pars tuberalis, also called the rostral tip, a region where the first alpha-glycoprotein subunit-expressing cells appear. Coimmunolocalization studies reveal that alpha-glycoprotein subunit-positive cells express the highest levels of Ptx1 throughout development and in the adult gland. The quantitative differences in Ptx1 expression in pituitary cell lineages may relate to a role in cell proliferation, lineage commitment, and/or the control of organ development.