In microsomes of control Wistar rats, NADPH-dependent reduction of 1-piperidinoanthraquinone (1-PA) to the corresponding hydroquinone, in the absence of oxygen, has been observed. Two facts ((i) inhibition of the formation of 1-piperidinoanthrahydroquinone (1-PAH) by metyrapone and antibodies to cytochrome P-450, and (ii) increase in the rate of 1-PAH formation upon induction of rats by phenobarbital) indicate that cytochrome P-450 participates in the reduction of 1-PA. Since 1-PA is a substrate of cytochrome P-450 and is oxidized in microsomes to (N-anthraquinonyl-1)-delta-aminovaleric acid (AAV), model experiments have been conducted to examine whether the reduced forms of 1-PA are involved in its oxidation. During photochemical generation of 1-PAH and its subsequent oxidation (N-anthraquinonyl-1)-delta-aminovaleric aldehyde (AAVal) is formed. However, this product is formed without participation of activated form of the substrate and oxygen. AAVal is a substrate in photochemical systems, apparently, is a precursor of AAV in microsomal oxidation of 1-PA. AAVal is substrate of cytochrome P-450 (the Type 1 of binding) and is oxidized quantitatively in microsomal systems to yield AAV. The date obtained enable us to propose a possible mechanism of enzyme oxidation of 1-PA.