AbstractThe effect of some 1,3,5‐triazine herbicides, on the rate of oxidative decarboxylation of tryptophan by peroxidase from corn seedlings, varied with the concentration. Atrazine was stimulatoryin vivoat 10‐6M but not at 108M or lower, whereas terbutryne was inhibitory at 10‐6M but not at 10‐8M or below. Prometryne inhibited enzyme activity at 10‐6M and stimulated it at 10‐10M and at 1012M, while simazine caused stimulation at 10‐6M and inhibition at 10−10M and 10−12M. Atraton reduced enzyme activity at 10−6M and 10−8M but had no effect at 10−10M and 10−12M. When peroxidase activity was assayed in acetone‐dried powders from seeds, harvested from fields that had received typical atrazine applications, the quantities of tryptophan consumed were significantly less than the quantity determined in the control powder. From the results obtained, by incorporating dealkylated and 2‐hydroxy metabolites of atrazine at 10−6M into the culture solutions, it seems possible that the inhibition observed may have been due to the presence of 4‐amino‐6‐ethylamino‐1, 3, 5‐triazin‐2‐ol and/or 4, 6‐diamino‐1, 3, 5‐triazin‐2‐ol residues in seeds from treated plots, even if, in previously reporte