The fine structural localization of peroxidases, during ethylene-induced abscission, in flower pedicels ofNicotiana tabacumL. cv. Little Turkish has been investigated. Peroxidase activity has been cytochemically localized in the rough endoplasmic reticula of tobacco abscission zone tissue that had been fixed in glutaraldehyde and subsequently incubated in diaminobenzidine (DAB) medium with post-fixation in osmium tetroxide. Tissue pre-incubated in potassium cyanide, prior to incubation in DAB medium containing the inhibitor, demonstrates a marked reduction in the DAB staining reaction. However, tissue pre-incubated in aminotriazole (an inhibitor of catalase enzymatic activity), prior to incubation in DAB medium containing the inhibitor, does not show inhibition of the DAB staining reaction in the rough endoplasmic reticula. It is suggested that the rough endoplasmic reticula, induced in response to ethylene treatment, exhibits enhanced peroxidase activity.