The pituitary-specific transcriptional factor, Pit-1, is a member of the POU-domain family which has a role in the development and differentiation of three pituitary cell types: somatotrophs, lactotrophs, and thyrotrophs. Recent investigations have suggested the involvement of specific regulation of Pit-1 transcripts in human pituitary adenomas. In this study, we analyzed the expression of Pit-1 gene and Pit-1 product in various human pituitary adenomas using in situ hybridization (ISH) and immunohistochemistry (IHC). Northern hybridization analysis revealed 2.4- and 4.1-kb Pit-1 transcripts in normal pituitary, growth hormone (GH)-, prolactin (PRL)- and thyrotropin (TSH)-secreting adenomas. By ISH analysis, Pit-1 mRNA was detected in 42 (84%) of 50 adenomas. The highest incidence was observed in 15 GH-secreting adenomas and 8 TSH-secreting adenomas, in which Pit-1 mRNA was detected in all cases. Pit-1 mRNA expression was detected in 11 (85%) of 13 PRL-secreting adenomas. In 12 clinically non-functioning adenomas, Pit-1 mRNA was also present in 8 cases, and 5 of these were associated with immunohistochemical expression of Pit-1 product. By combined ISH and IHC, Pit-1 mRNA was often colocalized with GH, PRL or TSH beta immunoreactivities and sometimes colocalized with alpha-subunit of glycoprotein (alpha SU) immunoreactivity. The expression of Pit-1 mRNA in various cell types of human pituitary adenomas in addition to GH, TSH beta and PRL immunoreactivities suggests that Pit-1 may play a role in functional development of pituitary adenomas, including clinically non-functioning adenomas. However, some additional transcriptional factors or enhancers may be required.