Abstract: Superoxide dismutase activity was detected in fourth instar Pieris rapae lar vae, and effects of the two chemicals, α -terthienyl and compound No.5, nam ely 1-phenyl-4-(3,4-methylene dioxy) phenyl-diacetylene, on superoxide dism utase activity, were compared under the specified condition. Results indicated t hat the near ultraviolet irradiation (300～400 nm) could enhanced the superoxide dismutase activity both in vivo and in vitro in control l arvae. After treated by the near ultraviolet irradiation and the two kinds of co mpounds at the same time, the superoxide dismutase activity in vivo could be decreased by α -terthienyl but not affected by compound N o.5. The superoxide dismutase activity in vitro was hardly affected by compo und No.5 whenever irradiation or not. Lower dosage of α -terthienyl could decrease the superoxide dismutase activity in vitro while higher dose could increase the activity, showing different relationship between superoxide dismuta se activity in vitro and dosage.It suggested that the larvae treated by α -terthienyl were more susceptible to light in vitro or in vitro . The pos sible mechanisms of action of the two compounds were discussed.