Superoxide dismutase (SOD) activity, the number of free radicals and pathomorphological changes were studied in lung and liver tissues of white rats with experimental paraquat poisoning. It was shown that SOD activity in the lungs and liver decreased and the number of free radicals rose. The inhibition of SOD activity after repeated administration of paraquat (1/10 of LD50) was more pronounced than in acute poisoning. Dystrophic and proliferative histomorphological changes characteristic for acute and chronic experiments and disturbed vascular permeability were revealed. It is suggested that the toxic action of paraquat on the animals is particularly consequent on the lack of SOD activity and enhancement of free radical processes.