Probenecid was accumulated by renal tissue both under aerobic and anaerobic conditions. The aerobic uptake at a low medium concentration of probenecid was enhanced three- or fourfold by acetate, while metabolic inhibitors and organic anions like p-aminohippurate (PAH), phenol red, and other substituted phenolsulphonphthalein (PSP) dyes which undergo secretion had an inhibitory effect. Octanoate and succinate stimulated aerobic probenecid uptake at low medium concentrations, but inhibited transport of the drug at high levels. Studies on renal homogenates showed that the anaerobic uptake of probenecid is due to binding to tissue constituents. Binding characteristics of PSP dyes to phospholipid vesicles and a representative binding protein, human serum albumin, exhibited close similarity to that of binding to renal tissue. Hydrophobic compounds like octanoate and PSP dyes partially inhibited probenecid binding. In contrast, transport of probenecid was almost completely abolished by these substances and PAH at high medium concentrations. On the basis of the results presented in this paper, it is concluded that renal transport of probenecid occurs by the common organic anion transport system.