Testosterone at doses of 0.1 mg/kg and 1 mg/kg was shown to affect the phosphohexose isomerase and aldolase activities as well as the ratio of individual isozymes of aldolase. Phosphohexose isomerase from liver tissue was especially sensitive towards the effect of the hormone. The activity of phosphohexose isomerase was selectively inhibited by testosterone. Activity of aldolase from seminal vesicles was practically unaltered. Four fractions of aldolase were found in rat liver tissue, two fractions--in seminal vesicles. Testosterone at a dose of 1 mg/kg inhibited the aldolase of the B type in liver tissue. The activity of aldolase of type A was inhibited only after a single hormone administration.