The activity of the p-glycoprotein (pGP) is an important factor for the multidrug resistant phenotype of cancers. Since solid-growing tumors often show pronounced extracellular acidosis, this study attempted to analyze the impact of an acidic environment on the activity of pGP and the cytotoxicity of chemotherapeutic agents. For this, prostate carcinoma cells (R-3327-AT1) were exposed to an extracellular pH of 6.6 for up to 24h. The pGP-activity more than doubled after 3 to 6h incubation in the acidic medium whereas the cellular pGP-expression remained almost constant, indicating that the increased transport rate results from a functional modulation. In parallel, the cytotoxic efficacy of daunorubicin showed a pronounced reduction at the low pH, an effect which was reversible upon co-incubation with a pGPinhibitor. A reduction in the intracellular Ca²⁺-concentration of 35% under acidic conditions induced a higher pGP transport rate, an effect which was comparable to that found upon inhibition of protein kinase C (PKC). These data indicate that the pGP-activity is increased as a result of lower extracellular pH presumably due to a low intracellular calcium level and an inhibition of the PKC. These findings may explain the reduced cytotoxicity of chemotherapeutic agents in hypoxic/acidic tumors. B.G., G.S., M.G.: Institute of Physiology, University of Wuerzburg