The role of serine endopeptidase in cucumber leaf senescence was studied by using the inhibitor of serine endopeptidase and plant growth regulators (6-BA and ABA) on darkness-induced cucumber leaves. The results showed that the senescence of cucumber leaves were delayed by AEBSF [4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride], an inhibitor of serine-type endopeptidase, or 6-BA treatment. The chlorophyll contents increased by AEBSF (Fig.3) and the protein degradation of leaves under AEBSF treatment declined more slowly than in the control or under ABA 50 micromol/L treatment (Fig.4), partly because the activities of serine endopeptidases became lower during senescence. However, the activities of endopeptidase in cucumber leaf were increased by ABA 50 micromol/L (Fig.2A), furthermore, the MDA content were also influenced by AEBSF and plant growth regulators (Fig.5). Native gradient PAGE showed that six bands of isoenzymes were detected in cucumber leaves and four bands of which were the type of serine-endopeptidase (Fig.1), and proved that the activities of serine-endopeptidase were inhibited by AEBSF, but enhanced by ABA (Fig.2B) in the leaves. It implies that serine endopeptidases might play an important role in cucumber leaf senescence.