The role of ACTH in stimulating or inhibiting growth of adrenal cells has been a subject of some controversy. Reports that ACTH may stimulate ERK/MAP kinase in Y1 cells have suggested a role for cAMP in this process. In attempting to extend this work, the ACTH responses in the human H295R cell line have been studied. This cell line makes only a very modest cAMP response to ACTH, yet the ERK1/2 response is highly reproducible and immediate, but not prolonged. It is minimally reduced by the protein kinase A inhibitor, H89, but unaffected by PKC and calcium inhibitors. Inhibition of EGF receptor or other tyrosine kinase receptor transactivation was without effect, as was inhibition of c-Src activity or c-Src phosphorylation. The most effective inhibitor of this pathway was dansylcadaverine, an inhibitor of receptor internalization. These findings imply that ACTH-induced ERK1/2 activation in H295R cells is dependent on a mechanism distinct from that by which most G protein-coupled receptors activate ERK1/2, but which nevertheless seems to depend on receptor internalization.