The effect of two methyluracil forms on the composition of lipids, the content of peroxidation products and the amount of lipid-soluble scavengers of free radicals was studied in rat liver and heart tissues. Both these forms caused an increase in content of total lipids in the animal liver tissue 2 hrs after injection, whereas beta-form increased 1.6-fold the content of cholesterol esters and 1.9-fold that of triglycerides in liver tissue, unlike the alpha-form which affected these parameters slightly. The effect of methyluracil polymorphic derivatives on lipid peroxidation (TBA-positive products) was not detected. The methyluracil beta-form, not affecting the total pool of hydrophobic antioxidants, was found to decrease the content of active scavengers of free radicals in rat liver and heart tissues, as well as beta-form decreased the content of vitamin E in these tissues. At the same time, the methyluracil alpha-form did not affect the composition of tissue hydrophobic antioxidants. These differences found in the effect of two methyluracil forms on lipid composition and lipid-soluble scavengers of free radicals may be responsible for biological nonequivalence of the drug polymorphic forms.