The active secretion of organic ions by the kidney may be described by the following models: 1)binding to a carrier protein and 2) a translocation process across the membrane. The feasibility of such a model was tested by measuring binding of either an organic cation, N1-methylnicotinamide (NMN) or an organic anion p-aminohippuric acid (PAH) to particulate material obtained from dog renal cortex tissue. The method employed was one in which the bound and free forms of the ligand were separated by centrifugation through a gel matrix. Binding of NMN and PAH was found to be tissue specific. In addition, binding was pH, time, temperature, protein-concentration and ligand-concentration dependent. Saturation of binding for either ligand was observed at concentrations greater than 50 mM, suggesting low affinity. Interestingly, a positive cooperative effect was observed for binding of either NMN or PAH to the particulate material. Although binding was associated only with particulate material, the binding proteins were released from the membrane system(s) by treatment with the nonionic detergent Lubrol WX. These studies show that NMN and PAH binding share many features in common but that the two processes are independent of each other. The results are consistent with, but do not prove, the model.