Pituitary adenomas have been shown to be clonal in origin, indicating that one or more somatic mutations underlie tumor pathogenesis. Mutated oncogenic forms of ras protein have been identified in a number of human neoplasms, including thyroid adenomas and carcinomas. However, the potential role of activated ras in the development of specific human pituitary tumor phenotypes has not been determined. Although ras mutations were not found in glycoprotein hormone-secreting or somatotroph adenomas, we recently identified a mutation in the H-ras gene (Gly-Val) at codon 12 in a highly invasive prolactinoma. These data raise the possibility that ras mutations might play a role in the pathogenesis of PRL-secreting pituitary tumors and/or may be a marker for tumor invasiveness and malignant transformation. Therefore, we investigated 78 pituitary tumors (59 prolactinomas, 13 invasive prolactinomas, and 6 pituitary carcinomas) for activating point mutation in the three ras genes using oligonucleotide-specific hybridization. In contrast to the relatively high frequency of ras mutations in many different tumor types, no ras mutations were identified in either prolactinomas or pituitary carcinomas. Our data indicate that ras mutations are rare in prolactinomas and pituitary carcinomas.