Metabolism of chloroquine in microsomes of mice liver tissue was studied in vitro by means of thin-layer and microcolumn liquid chromatographies. Chloroquine was metabolized in the system with liver microsomal monooxygenases. The products of its oxidation, involving side chain and aminoquinoline nucleus, were studied. As the enzymatic system is principally similar in various species, analogous metabolites of chloroquine appear to be produced after oxidation of the substance in the plasmodium microsomal monooxygenase system. Thus, this metabolic pathway is responsible for chloroquine resistance of the malarial parasite.