1 The anionic drug probenecid has been traditionally used as an inhibitor of renal organic anion transport. More recently the drug was found to inhibit organic cation transport as well, and it is used to retain intracellularly loaded¯uorophores. In these investigations it is implicitly assumed that probenecid performs its activity through competition for transport. Here we studied the possibility that probenecid provokes its e ect through inhibition of cellular oxidative metabolism. 2 Oxygen consumption was measured in isolated rat kidney cortex mitochondria. At concentrations of 1 mM or higher, probenecid increased the resting state (state 4) and decreased the ADP-stimulated respiration (state 3). A complete loss in respiratory control was observed at 10 mM probenecid. 3 After incubating isolated rat kidney proximal tubular cells (PTC) for 30 min with probenecid a concentration-dependent reduction in ATP content was observed, which was signi®cant at concentrations of 1 mM and higher. Using digital image¯uorescence microscopy the membrane potential in PTC was measured with bisoxonol. The mitochondrial e ects of probenecid were paralleled by a depolarization of the plasma membrane, immediately after drug addition. 4 All events are likely to be a result of membrane disordering due to the lipophilic character of probenecid, and may explain, at least in part, the various inhibitory e ects found for the drug. We recommend to be cautious with applying probenecid in cellular research.