Changes in the fine structure of flax (Linum usitatissimum) cotyledon leaf cells treated with the herbicide paraquat (1,1'-dimethyl-4,4'-bipyridylium ion.) were investigated. After 6 h treatment under constant illumination, tonoplast breakdown was evident. This was followed by a rapid and progressive deterioration of cell contents including the disruption of mitochondria, and the breakdown of chloroplast thylakoid membrane structure with the accumulation of osmiophilic plastoglobuli. There was no apparent deterioration after 18 h paraquat treatment in darkness but by 30 h there was a limited breakdown of chloroplast membranes.