1. Starvation for 3 days produces a decrease in methaemoglobin-reductase and glutathione-reductase activities, but it does not alter the glucose 6-phosphate-dehydrogenase activity of the rat erythrocyte. 2. The feeding of a protein-free diet for 11 days causes greater changes in the first two enzymes and also a diminution of the third. Under this experimental condition slight decreases in protein and haemoglobin contents were noted. 3. The experimental animals did not show methaemoglobinaemia, probably because the activity of methaemoglobin diaphorase is preserved. 4. The GSH content was not affected but the stability of the tripeptide in the presence of an oxidizing agent was diminished.