Effects of $alpha$-difluoromethylornithine (DFMO) and $alpha$-difluoromethylarginine (DFMA), inhibitors of polyamine syntheic enzymes (ornithine decarboxylase, arginine decarboxylase), on ovary were investigated during pupal-adult development of Helicoverpa assulta. Two inhibitors (DFMO, DFMA) showed definite inhibition effects on ovarian development. The inhibition effect on ovaries weight was more marked in DFMA-injection than that observed in DFMO-injection. Two inhibitors (DFMO, DFMA) gave rise to a peculiar decrease in ornithine decarboxylase (ODC) or arginine decarboxylase (ADC) activity in ovaries, 72 hrs (5-day old pupa) post injection. However, DFMO clearly exhibited supression of ODC activity after 96 hrs (6-day old pupa). In addition, two inhibitors (DFMO, DFMA) diminished putrescine content in the ovary. The spermidine level was slightly decreased by each injection of the inhibitors. However, two inhibitors (DFMO, DFMA) raised the spermine content at certain developmental stages in the ovary. Although the effect of DFMA was less severe, two inhibitors (DFMO, DFMA) caused not only an overall delay in ovarian development, but also abnormalities in cellular differentiation. Noted effects in the pupal ovary were the appearance of irregular nurse cells and partial destruction of follicle epidermal cells. Adult ovary showed rapid degradation of nurse cells, a reduction in the number of follicle epidermal cells and immature oocytes that had a low yolk content.