CYCLIC AMP¹ is thought to act as an intracellular second messenger, mediating the physiological response of many cell types to extracellular signals^2,3. In the pituitary, growth hormone (GH)-producing cells (somatotrophs) proliferate and produce GH in response to hypothalamic GH-releasing factor^4-8, which binds a receptor that stimulates G_s protein activation of adenylyl cyclase^3,9-12. We have now determined whether somatotroph proliferation and GH production are stimulated by cAMP' alone^5,7,11,13-15, or require concurrent, non-G_s-mediated induction of other regulatory molecules by designing a transgene to induce chronic supraphysiological concentrations of cAMP in somatotrophs. The rat GH promoter^16,17 was used to express an intracellular form of cholera toxin¹⁸, a non-cytotoxic and irreversible activator of G_s (ref. 19). Introduction of this transgene into mice caused gigantism, elevated serum GH levels, somatotroph proliferation and pituitary hyperplasia. These results support the direct triggering of these events by cAMP, and illustrate the utility of cholera toxin transgenes as a tool for physiological engineering.