Effect of intravenous administration of protoporphyrin IX (PP) on lipid peroxidation was studied in rats. PP and/or PP-derived porphyrins were found to be mainly distributed in livers, spleen and lungs. Dose-dependent decreases in the Fe²⁺ and L-ascorbic acid-stimulated lipid peroxidation in homogenates of livers and dose-dependent increases in porphyrin concentration in livers were observed after the PP injection. In the experiments with a 20 mg/kg dose of PP, the peroxidation level in the liver homogenates reached its minimum level during the period of 3 to 24 h accompanying the high porphyrin concentration in livers after the administration. After 96 h, a relatively high porphyrin concentration was still retained, but decreases in the peroxidation levels had ceased. PP administration caused a dose-dependent decrease in the endogenous lipid peroxides in livers within 0.5 h and the low levels were maintained throughout the course of the 168-h study. These results clearly show that the administered PP is distributed in the liver and inhibits the lipid peroxidation in vivo.