Decreased catalase activity is a consistent feature of leaf senescence. Although not as well documented, superoxide dismutase appears generally to decrease during leaf senescence. These changes suggest that free radical levels are likely to be higher in senescing tissues. The hydrogen peroxide-scavenging ability of chloroplasts due to the activity of the enzymes ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase appears to be established although there is no information on changes in levels of these enzymes in response to leaf senescence. In plants, unlike mammals, the direct reaction of glutathione with H₂O₂, catalysed by glutathione peroxidase, appears to be only a minor means of scavenging hydrogen peroxide. Senescence appears to be correlated with increases in lipid peroxidation and membrane permeability. The findings reviewed in this paper lend general support to the view that free radicals play a significant role in the multifactorial syndrome which constitutes leaf senescence.