Thirty-six rabbits received the herbicide, paraquat (1,1'-dimethyl-4,4'-dipyridylium dichloride), by selective intrabronchial instillation in doses ranging from 0.1 gm to 1 pg. Sixteen additional rabbits were given paraquat dichloride (25 mg/kg of body weight) intraperitoneally, subcutaneously, intravenously, or orally. The lung of the rabbit proved to be highly sensitive to paraquat administered by intrabronchial deposition, moderately sensitive to intravenously administered paraquat, and resistant to the chemical when given intraperitoneally or subcutaneously. One picogram of paraquat dichloride delivered by bronchial catheter produced a 1.0-mm pulmonary lesion consisting of focal hemorrhage, congestion of capillaries, intra-alveolar macrophages, and moderate thickening of the alveolar septa. This finding suggests that pulmonary damage could potentially result from smoking marihuana that has been sprayed with paraquat.