The activities of the enzymes of the urea cycle were measured in rat liver after feeding with a diet containing ethionine for 2 weeks. This treatment resulted in a decrease in the activity of ornithine transcarbamoylase, which fell to half the control value, while arginase increased by about 40%. The remaining enzymes of the urea cycle were unchanged at this time of treatment. These results are discussed in relation to the known effects of ethionine on nucleic acid synthesis. 2. The activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were measured in rat liver after feeding with a diet containing ethionine for 2 or 5 weeks. At the shorter time-period glucose 6-phosphate-dehydrogenase activity was more than doubled but 6-phosphogluconate-dehydrogenase activity remained unchanged. 3. The results are compared with similar measurements made after feeding with diets containing 4-dimethylamino-3'-methylazobenzene for 2 weeks. Striking similarities were found in the pattern of change in the enzyme activities of each of the pathways in the two treatments.
