Resistance to cytotoxic agents is a major problem in treating cancer. The mechanisms underlying this phenomenon appear to take advantage of functions involved in the control of cell homeostasis. A mechanism of resistance may be alteration of the tumour microenvironment via changes in the pH gradient between the extracellular environment and the cell cytoplasm. The extracellular pH of solid tumours is significantly more acidic than that of normal tissues, thus impairing the uptake of weakly basic chemotherapeutic drugs and reducing their effect on tumours. An option to revert multi-drug resistance is the use of agents that disrupt the pH gradient in tumours by inhibiting the function of pumps generating the pH gradient, such as vacuolar H(+)-ATPases (V-H(+)-ATPases). V-H(+)-ATPases pump protons across the plasma membrane and across the membranes of various intracellular compartments. Some human tumour cells, particularly those selected for multi-drug resistance, exhibit enhanced V-H(+)-ATPase activity. A class of V-H(+)-ATPase inhibitors, called proton pump inhibitors (PPIs), have emerged as the drug class of choice for treating patients with peptic diseases. These drugs inhibit gastric acid secretion by targeting the gastric acid pump, but they also directly inhibit V-H(+)-ATPases. PPIs (including omeprazole, esomeprazole, lansoprazole, pantoprazole and rabeprazole) are protonable weak bases which selectively accumulate in acidic spaces. Recent findings from our group have shown that PPI pretreatment sensitised tumour cell lines to the effect of cisplatin, 5-fluoro-uracil and vinblastine. PPI pretreatment was associated with the inhibition of V-H(+)-ATPase activity and an increase of both extracellular pH and the pH of lysosomal organelles, consistent with a cytoplasmic retention of the cytotoxic drugs and targeting to the nucleus in the case of doxorubicin. In vivo experiments showed that oral pretreatment with omeprazole induced a sensitivity of the human solid tumours to anticancer drugs.