In rats with experimental encephalopathy caused by intoxication with 4-pentenoic acid inhibiting beta-oxidation of medium- and long-chain fatty acids, hepatoprotector silimarin inhibited LPO, prevented deenergization and maintained high respiratory activity of brain mitochondria, and increased the rate and coupling of oxidation and phosphorylation. Succinic acid improved oxidation of substrates in motochondria and promoted activation of succinate-dependent ATP generation. Silimarin and succinic acid used together produced a synergistic protective effect on brain mitochondria surpassing the protective effects of individual preparations and prevented LPO activation.