1. The cysteine–cystine ratio was measured in rat kidney cortex, diaphragm, jejunum, liver and brain. 2. This ratio was determined by incubating these tissues in buffer containing [35S]cystine and then homogenizing the tissue in a buffered solution of N-ethylmaleimide. The products of this reaction were separated by high-voltage electrophoresis and the radioactivity in the cystine and 2-(l-2′-amino-2′-carboxyethylthio)-N-ethylsuccinimide regions was determined. 3. In these tissues cyst(e)ine was mainly present in the reduced form. 4. After incubation of [35S]cystine with rat jejunal segments it was found that 36% of the cystine in the medium has been reduced. 5. Anaerobiosis, Na+-free media, glucose and high concentrations of cystine and lysine were found not to affect significantly the cysteine–cystine ratio in rat kidney-cortex slices.
