AbstractChinese hamster embryo fibroblast cells (CHEF/18) possess a plasma membrane‐associated, amiloride‐sensitive Na+/H+ antiporter that affects intracellular pH (pHi) and is activated by growth factor addition. Our results using 14C‐benzoic acid distribution indicate that both epidermal growth factor (EGF) and thrombin are capable of causing rapid rises in the pHi of CHEF/18 cells. The maximal shift induced by these factors is 0.20 to 0.25 pH units above the basal unstimulated level. Distinctive differences were observed between the modes of action of these two growth factors. Sequential additions revealed that the rise in pHi due to EGF was additive with that caused by diacylglycerols (DAG), while that of thrombin was not. Furthermore, exposure of cells to the phorbol ester PMA for a prolonged period of time in order to down‐regulate protein kinase C (pkC), or treatment with the pkC inhibitor H‐7, abolished the pHi response to thrombin but not to EGF. In contrast, incubation of cells in nominally calcium‐free medium or with the calmodulin antagonists W‐7 or trifluoperazine (TFP) decreased only the ability of EGF to cause changes in pHi. These data suggest that there are two distinct mechanisms for activation of the Na+/H+ antiporter in CHEF/18 fibroblast cells and thus provide an example of the use of alternative modes for the modulation of intracellular processes.