Abstract: Water stress is one of the most important environmental factors that reduce growth, development and production of plants. Stress was applied with polyethyleneglycol (PEG) 6000 and water potentials were: zero (control), -0.15 (PEG 10%), -0.49 (PEG 20%), -1.03 (PEG 30%) and -1.76 (PEG 40%) MPa. The roots and leaves respiration of two maize (Zea mays L.) cultivars -704 and 301- were determined in various concentrations of PEG 6000. Oxygen uptake declined in leaves and roots with increasing PEG concentrations. Decrease of oxygen uptake in roots and leaves of 704 variety were higher than 301 variety. Chlorophyll a, b and total chlorophyll content were significantly decreased (p<0.05), but carotenoids content increased (p<0.05) under water stress. Decrease of chlorophyll content in 704 var. was higher than 301 var., but carotenoids content in 301 var. was higher than 704 var. Relative Water Content (RWC) was used to indicate the degree of stress. RWC decreased with increasing PEG concentrations. Lowering of RWC reduced growth and increased shoot/root ratio. Decrease of water content in 704 plants was higher than 301 plants. Shoot/root ratio in 704 var. was higher than 301 var.