Formation of sulfate in rat liver mitochondria was studied. About 0.1 mumol of sulfate was formed in mitochondria from 1 g of liver in 60 min when 10 mM L-cysteine was used as the substrate. Addition of either 10 mM 2-oxoglutarate or 10 mM glutathione to this system increased sulfate formation 3 to 4 times. The addition of both 2-oxoglutarate and glutathione resulted in a 20-fold increase in sulfate formation. Sulfate formation in the presence of 5 mM L-cysteine was 58% of that with 10 mM L-cysteine. L-Cysteine-glutathione mixed disulfide was not a good substrate, indicating that this mixed disulfide was not an intermediate of sulfate formation in the present system. Incubation of 3-mercaptopyruvate with rat liver mitochondria also resulted in sulfate formation, and the addition of glutathione accelerated it. Formation of sulfite and thiosulfate was also detected. These results indicate that sulfate is produced in mitochondria, at least in part, from L-cysteine through the transamination pathway (3-mercaptopyruvate pathway).