Abstract: Benzyladenine (BA) specifically promotes the expansion of excised cucumber cotyledons To investigate the mechanism of BA action, the fluctuation of endogenous zeatin (Z) and zeatin riboside (ZR) level was determined by indirect-ELISA and HPLC during the expansion process. The excised cucumber (Cucumis sativus, var Jin-ian No 4) cotyledons were incubated with 10 mg/1 of BA. Up to 72 hour, the fresh weight of the treated cotyledons increased more than 70% compared with that of control, large amount of Z+ZR were accumulated in the BA-treated cotyledons. This suggested that BA might trigger the cytokinin biosynthesis gene (s) in the cotyledons.