Summary1. Due to liver injury induced by carbon tetrachloride, the activities of GPT, GOT, arginase, tryptophan pyrrolase and δ4‐‐3‐ketosteroid reductase in rat liver and the ability of rat liver to inactivate estrogen decreased, while the activities of GPT and GOT in the brain were inversely increased. Decrease in the activities of GPT, GOT and arginase in the liver and the ability of the liver to inactivate estrogen were also to be brought on by forced running or immobilization.2. LSD increased the activity of hexokinase in the brain, but it had no effect on the enzyme activities of the liver or on estrogen inactivation by the liver.3. Stress conditions such as forced running, immobilization, exposure to cold, and the administration of chlorpromazine significantly increased the activity of tryptophan pyrrolase in rat liver. Removal of the adrenals did not affect the increase of enzyme activity, but ‘hypophysectomy’ completely abolished the enzyme induction.