Two animal models for cystinuria have been examined: the Basenji dog with Fanconi syndrome and cystine stone-forming dogs of various breeds. Brush-border membranes were isolated from these animals and uptake of D-glucose and L-cystine was characterized. Experiments with isolated brush-border vesicles from Basenji dogs with cystinuria as a component of the Fanconi syndrome showed diminished sodium-dependent D-glucose uptake but no decrease in L-cystine uptake even though the cystine defect in vivo was as high as 94% (ie, 6% reabsorption). In contrast, brush-border vesicles isolated from the kidney of a cystine stone-forming dog (Welsh Corgi) with a cystine defect of only 16% (ie, 84% reabsorption) had decreased uptake of cystine compared to values found for Beagle and Basenji vesicles. Thus, cystinuria found in Basenji dogs with the Fanconi syndrome differs from that in classic stone-forming cystinuric dogs. The alteration responsible for the cystinuria of Basenji dogs with Fanconi syndrome does not appear to have a membrane locus and may reflect altered energetics for transport, which are not detected in isolated vesicles. The cystine defect in cystinuric stone-forming dogs does appear to be reflected in the isolated membrane.