Dermal toxicity of paraquat in rats was studied, as well as its influence on internal organs, phagocytic and bactericidal activity of the neutrophile system, and the behaviour of the rats. The studies were conducted on 30 female rats of Wistar strain. The animals were divided into three groups, of which two groups were experimentally exposed to dermal absorption of paraquat (1/2 LD50 or 1/10 LD50), and one group was exposed as a control to dermal absorption of the solvent. The animals were exposed to dermal absorption for 4 hours daily for a period of 4 weeks. After 28 days, the rats were anaesthetized and blood was drawn from the heart to evaluate the activity of the neutrophilic system while the internal organs were excised for histological and ultrastructural studies. Histological and ultrastructural changes were observed in the lung, manifested by widened interalveolar septa filled with erythrocytes, lymphocytes, plasmatic cells, or alveolar macrophages. In the interalveolar septa bunches of collagen fibres were noted, while in the lumen of alveoli exudate and erythrocytes were observed. Histological and ultrastructural changes were also noted in the heart. They manifested themselves by focal hypertrophy of the interstitial tissue and by the increase of collagen fibres in bunches between cardiomyocytes. Less severe pathological changes were observed in kidney and liver. In the brain histological changes occurred in the neutrocytes. The bactericidal activity of the neutrophilic system increased in both experimental groups. Stimulation of phagocytosis was noted only in animals exposed to 1/10 LD50 - the lower dose of paraquat. The cognitive activity increased in both experimental groups after 2-weeks exposure to dermally applied paraquat, but returned to normal 2 weeks later.