We have shown that the small GTPase Rap1b, a protein known to antagonize the mitogenic and transforming activity of Ras, is endowed with both mitogenic and tumorigenic properties. Rap1b can be activated by cAMP, an intracellular message known to either stimulate or inhibit cell proliferation. The oncogenic property of Rap1b was revealed in a model system in which cAMP stimulates cell proliferation and was linked to Rap's ability to promote S phase entry. We have now tested the significance of the mitogenic action of Rap1b in a physiologically relevant model, the differentiated thyroid follicular cells, a system that requires thyroid-stimulating hormone (TSH), acting via cAMP, to mediate a full mitogenic response. Here we report that cAMP-dependent hormonal stimulation of DNA synthesis requires Rap1b in a manner dependent on its phosphorylation by protein kinase A.