Activity of main microsomal enzymes and the functional state of liver mitochondria were studied in male rats within 4, 7, 10 and 30 days after splenectomy. Within early periods after the operation content of cytochromes P-450 and b5, activities of amidopyrine N-demethylase, aniline hydroxylase and NADPH cytochrome c reductase were decreased, while the functional state of rat liver mitochondria was impaired. Administration of splenine into the animals within 6 days normalized distinctly the microsomal oxidation and reduced completely the liver mitochondria functional state in the systems involving both substrates of oxidation.