Migration and morphology of human melanoma cells (MV3) depend on extracellular pH (pHe) and the activity of the Na+/H+ exchanger NHE1. To distinguish effects of NHE1 activity per se from effects of pHe we compared an NHE1-deficient mutant with rescued and wild-type cells. Time lapse video microscopy was used to investigate migratory and morphological effects caused by pHe and NHE1 activity, and a membrane-bound fluorescein conjugate was employed for ratiometric pH measurements at the outer leaflet of the cell membrane. As long as NHE1 remained inactive due to deficiency or inhibition by cariporide (HOE642) neither migration nor morphology was affected by changes in pHe. Under these conditions pH at the outer leaflet of the plasma membrane was uniform all over the cell surface. The typical pH dependence of MV3 cell migration and morphology could be reconstituted by restoring NHE1 activity. At the same time the proton gradient at the outer leaflet of the plasma membrane with the higher proton concentration at the leading edge and the lower one at the cell rear was re-established as well. Hence, NHE1 activity generates a proton gradient at the cell surface accompanied by the cells' ability to respond to changes in pHe (bulk pH). We conclude that NHE1 activity contributes to the generation of a well-defined cell surface pH by creating a proton gradient at the outer leaflet of the plasma membrane that is needed for (i) the development of a variety of morphologies including a distinct polarity and (ii) migration. A missing proton gradient at the cell surface cannot be compensated for by varying pHe.