Objective: The p-glycoprotein (pGP) is the most prominent member of active drug transporters leading to a multidrug resistant phenotype. In vitro it has been demonstrated that the functional pGP-activity can be doubled by an extracellular acidosis (pH=6.5). Here MAP kinases (p38, ERK1/2) seem to play an important role for signal transduction. However, it is unclear whether these effects are also relevant in vivo. Methods: For imaging the pGP activity in PET a new ⁶⁸Ga-labeled tracer (the Schiff´base ligand complex [⁶⁸Ga]MF6.MZ) was developed which serves as a substrate of the pGP. Therefore, its intracellular concentration inversely reflects the pGP transport rate. For in vivo studies experimental tumors (R3327-AT1 prostate carcinoma of the rat) were imaged under acidic conditions (induced by inspiratory hypoxia (8% O₂) or by injection of lactic acid) and during inhibition of MAPKs in a m-PET system. Results: In solid tumors the tracer showed a marked uptake whereas direct intratumoral injection of verapamil increased the tracer concentration by ~25% indicating the changes in functional pGP-activity. With increasing tumor volume the tumor pH went down and in parallel the pGP activity was increasing almost linearly. Direct acidification of tumors by lactic acid injection reduced the PET-tracer accumulation by 20% indicating a higher transport rate out of the cells. Lowering the inspiratory O₂-fraction led to a reduction of the extracellular pH and in parallel to a reduced tracer concentration. Inhibition of the p38 pathway (however not the ERK1/2) reduced the pGP transport rate. Conclusion: The new ⁶⁸Ga-labeled tracer is suitable for PET-imaging of the tissue pGP-activity which allows to pre-therapeutically identify patients with multidrug-resistant tumors. Noninvase imaging reveals that an extracellular acidosis activates the pGP markedly presumably via the p38-MAPK pathway. From these results new strategies for overcoming multidrug resistance may be developed.