Pea (Pisum sativum L. cv. Frilene) plants subjected to drought (leaf water potential of ~ -1.3 MPa) showed major reductions in photosynthesis (78%), transpiration (83 %), and glycolate oxidase (EC 1.1.3.1) activity (44%), and minor reductions (~ 18%) in the contents of chlorophyll a, carotenoids, and soluble protein. Water stress also led to pronounced decreases (72 85%) in the activities of catalase (EC 1.11.1.6), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), but resulted in the increase (32~42%) of nonspecific peroxidase (EC 1.11,1.7) and superoxide dismutase (EC 1.15.1.1). Ascorbate peroxidase (EC 1.11.1.11) and monodehydroascorbate reductase (EC 1.6.5.4) activities decreased only by 15% and the two enzymes acted in a cyclic manner to remove H20 2, which did not accumulate in stressed leaves. Drought had no effect on the levels of ascorbate and oxidized glutathione in leaves, but caused a 25% decrease in the content of reduced glutathione and a 67% increase in that of vitamin E. In leaves, average concentrations of catalytic Fe, i.e. Fe capable of catalyzing free-radical generation by redox cycling, were estimated as 0.7 to 7 laM (well-watered plants, depending on age) and 16 pM (water-stressed plants); those of catalytic Cu were ~4.5 gM and 18 gM, respectively. Oxidation of lipids and proteins from leaves was enhanced two-to threefold under stress conditions and both processes were highly correlated. Fenton systems composed of the purported concentrations of ascorbate, H~O2, and catalytic metal ions in leaves produced hy-