In pituitary cells, activation of the cAMP pathway by specific G-protein-coupled receptors controls differentiative functions and proliferation. Constitutively active forms of the subunit of the heterotrimeric Gs protein resulting from mutations at codon 201 or 227 (gsp oncogene) were first identified in 30-40% of human GH-secreting pituitary adenomas. This rate of occurrence suggests that the gsp oncogene is not responsible for initiating the majority of these tumors. Moreover, there is a large overlap between the clinical phenotypes observed in patients with tumors bearing the gsp oncogene and those devoid of this oncogene. To explore the role of Gs in GH-secreting adenomas, we obtained somatolactotroph GH4C1 cell lines by performing doxycycline-dependent conditional overexpression of the wild type Gs protein and expression of the gsp oncogene. Although the resulting adenylyl cyclase and cAMP levels were ten-fold lower in the wild type Gs overexpressing cell line, a sustained MAPKinase ERK1/2 activation was observed in both cell lines. Overexpression of the wild type Gs protein as the gsp oncogene initiated chronic activation of endogenous PRL synthesis and release, as well as chronic activation of ERK1/2-sensitive human PRL and GH promoters.