The influence of 1, 2-alkylene-l, 2, 4-triazolidines and 3, 4-alkylene-1, 3, 4-thiadiazolidines on peroxidizing phytotoxicities was investigated using corn etioplast protoporphyrinogen-IX oxidase (Protox), autotrophic Scenedesmus acutus cells and Echinochloa utilis. Both types of compounds inhibited Protox, caused ethane formation, decreased chlorophyll content and inhibited growth of Scenedesmus and root-growth of Echinochloa, like peroxidizing herbicides such as p-nitrodiphenyl ethers and cyclic imides. All compounds were identified as peroxidizing herbicides. Among the 1, 2-alkylene-l, 2, 4-triazolidines, tetramethylene-triazolidines exhibited the strongest peroxidizing phytotoxicity, trimethylene-triazolidines and pentamethylene-triazolidines were found to be less active peroxidizers. The tetramethylene moiety of the triazolidine-type compounds appears to be essential for producing active peroxidizing compounds.Phytotoxicities by triazolidin-one-thiones and triazolidine-dithiones were more than 10 times stronger than those of triazolidine-diones.Replacing carbonyl groups in the five-membered moiety by at least one thiocarbonyl group produces stronger peroxidizing phytotoxities. Although the Protox inhibition of triazolidine-peroxidizers was 10-70 times stronger than that of the corresponding thiadiazolidineperoxidizers, the level of other phytotoxic activities was quite similar with both types of peroxidizers. This fact may be caused by isomerization of thiadiazolidines to triazolidines.