The insect neuropeptide, leucokinin it was injected into larvae of the cotton leafworm Spodoptera littoraiis(Boisd.) to investigate the influence on development, digestive enzyme release and heamolymph main component. One injected dose of (2x166 to 4x10'aM) of leucokinin ll into the 5"" instar larvae induced prolongation in pupal period in all experimental groups, while it had no effect on the timing of larval ecdysis and length of feeding period. Also leucokinin ll caused a great reduction in pupal weight, adult longevity, oviposition period, fecundity and fertility. A significant increase in larval mortality 66.66 to 86.66% was observed. Leucokinin ll inhibited the release of digestive. protease, amylase, trehalase and invertase. A significant reduction was obtained in total protein level, whereas total carbohydrate and total lipid levels were significantly increased in dose dependent manner.