Excised leaves of kidney bean (Phaseolus vulgaris) were used to investigate the mechanism of NO generation under UV-B stress. We showed that two signaling molecules, NO and $H_2O_2$, were produced in the irradiated leaves. NO release was blocked by LNNA, an inhibitor of NOS. Application of CAT (EC 1.11.1.6) not only effectively eliminated $H_2O_2$ in the leaves, but also inhibited the activity of NOS and the emission of NO. In contrast, treatment with exogenous $H_2O_2$ increased both of those events. Therefore, we suggest that, under UV-B stress, NO production is mediated by $H_2O_2$ through greater NOS activity.