Rats were injected 21 mg/kg dimethylhydrazine (DMH), in a single dose, subcutaneously (experimental run I), or 5 times at weekly intervals (experimental run II) to measure free-radical process indices in blood serum, liver and large bowel 24 hr after a single exposure and 6 months after 5 exposures. Run I identified marked DMH inhibition of blood-serum level of active forms of oxygen (AFO). However it did not involve any changes in the concentrations of such products of lipids peroxidation (LP) as dien conjugates, Schiff's bases and protein peroxides (PP). Neither did total antioxidation activity (TAA) or that of Cu, Zn-superoxydismutase (SOD) change. However, nitrite levels increased while those of ceruloplasmin decreased significantly. AFO levels also dropped in the liver, following a single injection of DMH, with TAA and SOD activity decreased significantly. No changes were observed in AFO generation processes in the bowel which was targeted by DMH treatment. A slight decrease in TAA, a rise in PP and diverse changes in the enzymatic system of antioxodation protection were recorded: decreased SOD activity and an insignificant rise in catalase level (like in the liver). Run II showed an intensification of peroxidation in blood serum and bowel in animals bearing intestinal tumors. However, no distinct changes occurred in the liver. The results point to a tendency of free-radical processes being intensified during DMH-induced carcinogenesis in rats.