Effects of dimethipin on the senescence of excised barley first leaves were investigated. Dimethipin markedly inhibited chlorophyll and protein loss and reduced peroxidase activity relevant to senescence phenomena in th excised leaves. Dimethipin decreased hydrogen peroxide content, later malondialdehyde content, and increased the activities of superoxide dismutase and catalase. The antisenescence effects of dimethipin may result from the stabilization of membrane structure through inhibiting the peroxidation of unsaturated lipid and the accumulation of free radicals during senescence.