Inhibition of rat liver transketolase under conditions of hydroxythiamin or alimentary B1 avitaminosis was accompanied by an increase in intracellular concentration of 6-phosphogluconate and xylulose-5-phosphate. The alterations in their content was inversely correlated with the transketolase activity. The dehydrogenases involved in the pentose phosphate shunt exhibited various unspecific sensitivity to hydroxythiamin. The doses of the antimetabolites up to 0.4 mg per rat did not alter the activity of the pentose phosphate shunt dehydrogenases; an increase in the dose from 4 up to 20 mg led to an inhibition of the enzymes. Possible biochemical alteration, occurring in response to various degree of transketolase inhibitions, are discussed.