The feeding of crude, thiaminase-containing herring to rainbow trout produced external symptoms of thiamine deficiency after a period of 150 days. The level of transketolase activity in the kidney decreased continuously until the first appearance of deficiency symptoms, while the TPP effect showed an upward trend. After prolonged feeding of crude herring the decline of transketolase activity is accompanied by a deficiency of the apoenzyme of transketolase. The administration of thiamine-containing food to the thiamine-deficient animals induced an immediate increase n transketolase activity. Once the external deficiency symptoms are very pronounced the death of the animal can only, if at all, be prevented by an injectionof thiaminie.