This experiment was carried out to evaluate the effects of synergists on toxicity of deltamethrin and to investigate the pharmacokinetics of deltamethrin in Helicoverpa assulta Guenee and spodoptera litura Fabricius. Susceptibilities of H. assulta and Haman-collected S. litura to 7 insecticides revealed different responses between the 2 species. The different susceptibility of the 2 species to deltamethrin was correlated with the differences in the penetration and metabolism. Larvae of H. assulta pretreated with synergists (piperonyl butoxide (PB), S-benzyl-O, O-diisopropyl phosphorothioate (IBP), and diethyl maleate (DM)) were more sensitive to deltamethrin, with the highest synergism ratio (SR=6.8) being observed with PB. Significant differences in synergism among synergists were not observed. For S. litura, the highest SR(=8.8) was observed with PB. At all time periods, the penetration of ${14}^{C}$-deltamethrin was relatively faster to S. litura than to H.assulta. Time (hrs) required for 20% (${t}_{20}$) of the applied doses to be excreted from treated S. litura was 1.5-fold faster than that of H. assulta. Higher synergism and longer retention in larval body of S. litura by the addition of PB suggest that the mixed function oxidase be a major factor in detoxification of deltamethrin. The combined use of PB with deltamethrin, and possibly other pyrethroids, might indicate a potential for controlling field populations resistant to deltamethrin and other pyrethroids.