Self-exchange of chloride and sulfate in dog and cat red cells has been measured under equilibrium conditions. The rats of efflux for these anions are approximately twofold higher in dog compared to cat red blood cells. Although the rates differ, the anion exchange systems of these two red cell types exhibit many common properties. The dependence of 35SO4 efflux on the intracellular SO4 concentration, the pH dependence and the inhibition of 35SO4 efflux by Cl and SITS are almost identical in dog and cat red cells. Nystatin treatment was used to study the dependence of 36Cl efflux on internal Cl. Chloride efflux exhibits saturation in both cell types with dog red cells possessing a higher Vmax and K1/2 than cat red cells. The number of anion transport sites was estimated by extrapolation to the number of molecules of dihydro DIDS (H2DIDS, where DIDS is 4,4'-diisothiocyano-2,2' stilbene-disulfonic acid) which were bound at 100% inhibition of transport. The results indicate that either the turnover numbers for anion transport differ in dog, cat, and human red cells or that there is heterogeneity in the function of the membrane components which bind H2DIDS.