Upregulation of proton extrusion is critical for tumor cell survival in an ischemic microenvironment with a lower extracellular pH (pHe). Lower pHe and higher intracellular pH (pHi) benefit cancer cells for invasion and growth. Vacuolar H(+)-ATPases (V-H(+)-ATPases) play a critical role in regulating the transmembrane pH gradient. Proton Pump Inhibitors (PPI), mainly treating acid-related diseases, could inhibit the expression of V-H(+)-ATPases. We have investigated whether PPI decreases the pHi of the human gastric adenocarcinoma cell line, SGC7901, by inhibiting V-H(+)-ATPases so as to enhance the cytotoxicity of anti-tumor drugs. We have assessed the optimal treatment time, pretreatment dosage of PPI and the possible mechanism of action. PPI exceeding 10 microg/ml inhibited protein expression of V-H(+)-ATPases in a dose-dependent manner, decreased the pHi value and reversed the transmembrane pH gradient, whereas PPI at final concentration of 1 microg/ml could not. Changes of the pH gradient were positively correlated with PPI concentration. The inhibitory effects of PPI on V-H(+)-ATPases primarily occurs from 12h to 24h after PPI pretreatment (P<0.05). The pHi value of SGC7901 was lowest 24h after PPI pretreatment (P<0.05). Administration of anti-tumor drugs 24h after PPI pretreatment produced the most cytotoxic effects on SGC7901 (P<0.05) and significantly improved the early and total apoptosis rates (P<0.01). PPI exceeding 20 microg/ml also significantly reduced the ADR-releasing index, thereby enhancing the intracellular ADR concentration (P<0.01). Therefore, PPI could enhance the cytotoxic effects of anti-tumor drugs on the SGC7901 cells.