Zearalenone, a natural product isolated from Fusarium graminearum, has anabolic properties and affects glycogen catabolism in the rat liver. In the isolated perfused liver, L-lactate production, glucose release from endogenous glycogen and oxygen uptake are increased upon infusion of the compound. Fructose metabolism is only slightly affected by zearalenone, except for the transformation of fructose into glucose which is decreased. The action of zearalenone seems to be related to the amount of drug retained by the liver rather than to the concentration in the perfusion fluid.