Studies were undertaken to evaluate the effects of mechanical impedance on root exudation by maize (Zea mays L., var Dea) and to examine the importance of these effects in relation tp the stage of plant development. Plants were grown under sterile and hydroponic conditions. Mechanical impedance was simulated using glass beads of 1 mm diameter. This treatment was compared with a control without beads. Results demonstrated that plant growth was influenced by mechanical jmpedance. Mechanical impedknce markedly affected the growth of the shoot, whether this was measured as leaf area or total dry matter. Besides increasing root/shoot bipmass ratios, mechanical impedances also stimulated root exudation of organic and inorganic Compounds. Stressed plants lost more nitrogenous compounds than control plants. Otherwise, the percentage of released carbon decreased. Depending on the developmental stage of the plant, there was a large variation in the magnitude and time course on mechanical impedance effects. The effects of mechanical impedance persist and accentuate with time.