Beneficial effects of low-concentration chemical stressors have been investigated previously in different model systems. The symptoms of stimulation are known from earlier studies, but information about the mechanism is at an initial stage. In the present work, the mechanism of stimulation of low-concentration Cd (5 x 10(-8)M) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, 10(-7)M) was investigated in barley seedlings. In treated plants, the amount of cytokinins increased in roots and, after being transported to the leaves, they caused stimulation there. To identify the signal transduction pathway(s) involved in the primary stimulation of cytokinin synthesis (and/or activation) in roots, specific phosphatidylinositol-4,5-bisphosphate-inositol-1,4,5-triphosphate/diacylglycerol (PIP(2)-IP(3)/DAG) and mitogen activated protein kinase (MAPK) signaling pathway inhibitors were added to the nutrient solution, and all proved to be effective, eliminating the stimulation by the stressors. Measurements of superoxide dismutase (SOD, EC 1.15.1.1) activity and the amount of malonyldialdehyde (MDA) showed that the increased amount of Cd did not cause oxidative stress in the roots, and no oxidative stress was found in the leaves, where Cd did not even accumulate. DCMU slightly increased the activity of SOD after 1 week in roots, but did not cause lipid peroxidation. In leaves, there was no oxidative stress upon treatment with DCMU. Thus, oxidative stress cannot be responsible for the stimulation with low-concentration stressors, as they changed the activity of SOD differently, while being equally stimulative for the plants.