Intoxication of rats by 0,0-dimethyl-0-2,2-dichlorovinylphosphate (DDVPh) leads to marked biochemical changes of transcriptionally active (TACh) and repressed (RCh) liver chromatin fractions. These changes are connected with structural relaxation of the fractions, which is manifested by the increase of intensity of protein self-fluorescence. Free radical nature of these changes was supposed, which was proved by the modification of LPO reactions in the both chromatin fractions under the influence of the pesticide concerned.