Lesions caused by a single subtoxic dosis of diquat were studied in rats. After the administration of diquat on the first day it was revealed in highest concentration in the lung, but in two weeks no traces of the drug could be demonstrated. On the first and third day activity of peroxidase in the liver and the number of peroxisomes as well significantly increases. On the first day after the administration of diquat in the lung damage to the surfactanct and increase of the capillary permeability was observed. Later degenerative changes of the pneumocytes I and II type, focal haemorrhagic edema and presence of alveolar macrophages can be revealed. In two and in four weeks a slight pulmonary fibrosis remains to show the damage to the lung. In kidney tubular lesions of toxic origin, in the phase of the regeneration increase of the number of lyosomes could be revealed. On the basis of this study and the data of other authors the possible mechanism of the intoxication with diquat is discussed.