Corticotropin-releasing hormone (CRH) plays an important role in regulating the development and function of hypothalamic-pituitary-adrenal axis. The mechanisms by which CRH regulates tissue-specific growth, differentiation and gene expression remain to be established. In the present study, we show that CRH differentially regulates MAP kinase activity in normal ovine anterior pituitary cells and mouse corticotrope AtT20 cells. Incubation of ovine normal anterior pituitary cells with CRH increased MAP kinase activity, an effect mimicked by cAMP and inhibited by the protein kinase A inhibitor H89. In contrast, incubation of mouse pituitary tumor AtT20 cells with CRH inhibited MAP kinase activity, an effect also mimicked by forskolin and inhibited by H89. This decrease in MAP kinase activity occurred with a time course similar to the increase seen in normal anterior pituitary cells. Furthermore, both effects of CRH on MAP kinase activity were inhibited by atrial natriuretic peptide (ANP). ANP also reversed the inhibition of DNA synthesis induced by CRH in AtT20 cells. Thus, CRH may differentially regulate cell growth in sheep normal anterior pituitary and mouse tumor corticotropes by modulating MAP kinase activity through a mechanism dependent on cAMP production and subject to regulation by ANP.