Large doses of cephaloridine cause acute necrosis of the proximal renal tubule that can be prevented by probenecid and other organic anions. Although there is little or no net secretion of cephaloridine by the mammalian kidney, the degree of cephaloridine uptake by the cortex of the rabbit kidney is substantial; this uptake is also prevented by probenecid and other organic anions. Cortical concentrations of cephaloridine were measured in control and probenecid-treated animals of different mammalian species. Evidence of cephaloridine trnsport was found in the guinea pig and the rat as well as in the rabbit. The degree of reduction of cortex-to-serum ratios by probenecid (control cortex-to-serum minus probenecid-treated cortex-to-serum ratios) correlated with the sensitivity to the nephrotoxicity of the drug. This degree or reduction was greatest in the rabbit, intermediate in the guinea pig, and least in the rat. In addition, the newborn rabbit, which is more resistant to the toxicity of cephaloridine than the adult, also had significantly lower cortical concentrations of cephaloridine. Finally, the acute tubular necrosis produced by extremely large doses of cefazolin in the adult rabbit was prevented by probenecid. It was concluded (1) that the nephrotoxicity of cephaloridine is related to its renal cortical transport with high intracellular concentrations of drug; and (2) that this relationship between transport and toxicity exists for cefazolin as well, although the toxicity is of a different order of magnitude. The unusual mechanism of cephaloridine transport in the proximal tubule was contrasted with that of the other cephalosporins in an attempt to explain its greater degree of nephrotoxicity.