Activities of glucose-6-phosphate dehydrogenase and transketolase from rat liver tissue exhibited circadian rhythms with the maximal rate at 22 and 6 o'clock and minimal--at 14 o'clock (per 1 mg of liver protein and per 1 g of the tissue). Decrease in total activity of glucose-6-phosphate dehydrogenase at the day time was due to distinctly lowered activity of the minor isoenzyme components. Differences in activity of the enzymes were more than 100% at the maximal and minimal periods. The data obtained earlier and in the experiment reported herein suggest that alterations in activity of pentosephosphate pathway enzymes, observed in the states of stress reactions, are presumed to be compensated if they are within the limits of extreme circadian variations; activity of the enzymes, exceeding the circadian variation values, indicates decompensation in the state of carbohydrate turnover.