Toxic doses of aminoglycosides augment organic anion transport in the rat kidney. Studies in rabbits and guinea pigs have shown that the nephrotoxicity of cephalosporins is closely related to their active transport by the organic anion carrier in the proximal renal tubule. We have examined the effects of the aminoglycosides neomycin and gentamicin on cephaloridine transport and cytotoxicity in the rabbit kidney. There was no effect of these aminoglycosides on renal cortical cephaloridine uptake. In several regimens of combined administration, there was also no additive nephrotoxicity of the aminoglycosides and cephaloridine, even though some degree of tubular necrosis was seen in the single drug control groups in each protocol. These results are in agreement with those of studies in the rat and provide evidence that the cellular mechanisms of nephrotoxicity of the cephalosporins and aminoglycosides are not closely related.