Isolated rat renal tubules were loaded with cystine by incubating them with 2 mM cystine dimethylester. The oxidation of 1 mM glucose and lactate was significantly decreased after 20 and 30 min of incubation, in the cystine-loaded tubules compared with control tubules. The oxidation of 1 mM butyrate was significantly decreased after 10 and 30 min of incubation in the cystine-loaded tubules. Cystine loading decreased the oxidation of 1 mM succinate at all time points examined. The O2 consumption of renal tubules was reduced 59% with cystine loading by the addition of 2 mM cystine dimethylester, and by 37% with 1 mM cystine dimethylester. These data indicate that loading normal renal tubule cells with cystine impairs their ability to oxidize metabolic fuels. This impairment in metabolism may explain the decreased transport observed previously in cystine-loaded tubules and may have implications for the human disorder, cystinosis.