The aim of this study was to test the hypothesis that pathogenesis induced in sensitive plants by ALS-and ACC-inhibiting herbicides is an active process which involves programmed cell death (PCD). A reliable marker of PCD is DNA fragmentation, which is carried out by endogenous nucleases. The root meristems from herbicide-sensitive plants were analyzed by the TUNEL assay to estimate DNA fragmentation. It was shown that the highest level of DNA fragmentation in root meristem cells under pathogenesis, induced on maize seedlings by the action of ACC-inhibiting herbicide propaquizafop, and on pea seedlings by the action of ALS-inhibiting herbicide tribenuron-methyl, was associated with increased total nuclease activity in root meristem lysates comparing to the corresponding control. In response to tribenuron-methyl application, the level of total nuclease activity elevated due to high activity of both Zn 2+ -dependent («acidic») and Ca 2+ /Mg 2+dependent («alkaline») nucleases, while in a case of propaquizafop, the increased activity was detected only for Zn 2+ -dependent nucleases. These differences may be due to different mechanisms of PCD initiation, depending on the mode of herbicides action. The facts, that TUNEL method recorded DNA fragmentation in plant cells induced by ACC-and ALS-inhibiting herbicides, and this fragmentation is associated with an increased activity of endogenous nucleases, represent evidence for PCD, that occurs during pathogenesis. The significance of data obtained is that they emphasize the fact that herbicide-induced pathogenesis is a complex, multi-stage, active process. The discovery of pathogenesis distinct stages mechanisms nature opens up new possibilities for regulating the herbicides selective phytotoxicity by physiologically active substances and genetically engineered manipulations.