Adult female Sherman strain rats were fed 500 ppm paraquat (1,1-dimethyl-4,4-bipyridylium dichloride) in their diet. Four poisoned as well as three control rats were sacrificed 1, 3, 5, 7 and 11 wk after onset of exposure. Two well-developed paraquat lung lesions were encountered. The alveoli of these lesions contained a very prominent amorphous material which was highly osmiophilic when examined with the electron microscope. It consisted of concentrically arranged whorls which were surrounded by or next to less electrondense lattice-like material. Additional ultrastructural findings in those and other lungs consisted of edematous and vacuolated membranous pneumocytes (type I) as well as endothelial cells. The granular pneumocytes were increased in 3 of 20 lungs of exposed animals.