The renal proximal tubule actively transports charged, potentially toxic xenobiotics from blood to lumen. Basolateral uptake of organic anions is indirectly coupled to the sodium gradient through Na-dicarboxylate cotransport and dicarboxylate-organic anion exchange. Upon entry, a significant fraction of intracellular organic anion is sequestered within vesicles. Disruption of the cellular microtubular network can lead to both diminished vesicular movement and reduced transepithelial secretion. Luminal efflux of organic anions is energetically downhill, but carrier mediated. Both anion exchange and potential driven transport are present, but neither completely accounts for transport from cell to lumen. For organic cations, basolateral entry is downhill via potential driven facilitated diffusion. Intracellular sequestration of organic cations in vesicles is substantial, but its role in secretion is uncertain. Multiple carriers are available to drive organic cations uphill into the tubular lumen. The classical system indirectly taps the energy of the luminal Na gradient to drive organic cation efflux via Na(+)-H+ and proton-organic cation exchange. In addition, the multidrug resistance ATPase can pump organic cations into the tubular lumen. Thus, although much detailed information has been added over the last 50 years, it is not yet possible to provide a detailed, quantitative understanding of these important excretory systems.