Four-week-old velvet beans (Mucuna pruriens L.) were fumigated with ozone, isoprene and propene both singly and in combination (hydrocarbon+oxidant) in controlled-environment chambers for a 4 week period. Measurements were made of total soluble protein, ascorbate, dehydroascorbate, lipid peroxidation, and glutathione reductase activity in mature and young velvet bean leaves.Significant increases in soluble protein concentrations with respect to the controls were found in plants fumigated with propene and isoprene+ozone for young leaves, and ozone only for mature leaves. The analysis of ascorbate concentrations in young leaves showed a significant increase in total ascorbate for propene-fumigated plants a significant decrease in the level of the oxidized form (dehydroascorbate) for ozone and isoprene+ozone-treated plants, and a significant increase in the reduced form for plants fumigated with propene, propene+ozone and isoprene+ozone. The analysis of ascorbate levels in mature leaves showed no significant effect from any fumigation regime.When compared to control material the specific activity of the enzyme glutathione reductase was found to be significantly reduced in young leaves treated with propene, isoprene+ozone and ozone alone. However, in mature leaves this effect is lost and there are no significant differences with respect to the control