The present study was investigated to elucidate the effects of chlorambucil the heart tissue of various-aged rats. The male rats ranging from 3 to 36 months were used. The cytochemical and biochemical changes in myocardium of the rats were studied in the aspect of free radical roles in aging process. With the goals of evaluating the potential roles of free radicals in aging process, evidence was shought for alterations of myocardial lipid peroxide levels in control and chlorambucil treated rats. The result are summarized as follows: 1. Cytochemical studies showed that the activities of $Mg^{++}$-ATPase and succinic dehydrogenase increased with age. However, these enzyme activities were decreased with treatment of chlorambucil, when compared with control group. Interestingly it was observed that chlorambucil treatment increased the activity of acid phosphatase from 6 months upto 18 months, and decreased after 18 months. 2. The lipid peroxide level in myocatdium was increased with age; chlorambucil-treated group was higher than that of control group. 3. Age-dependent increase in activities of monoamine oxidase, xanthine oxidase and catalase was observed. But the increase of catalase activity was higher than that of monoamine oxidase and xanthine oxidase activity in control group. However, in chlorambucil-treated group, age-dependent decrease of these enzyme activities was observed, and catalase activity was more significant particularly with regard to other enzymes. In consequently, the morphological alterationsof myocardium due to chlorambucil treatment was exclusively observed. We demonstrate that this alteration is occured by lipid peroxidation upon chlorambucil treatment.