In the corn earwormHelicoverpa zeayolk deposition in oocytes begins 8–10 h after adult eclosion and is controlled by juvenile hormone (JH). Hemolymph from females at the time of yolk formation, analyzed by SDS-PAGE, contains polypeptides apparently identical to polypeptides from eggs; and these have been tentatively designated as apovitello-genins (apoVgs). ApoVgs were not found in male hemolymph. The hormonal control of production of the apoVg with an apparent molecular weight of 165 kDa (apoVg-I) was studied by SDS-PAGE. Decapitation of females immediately after eclosion did not prevent the appearance of apoVg-I whereas the same operation done 5–7 h prior to eclosion abolished its production. Treatment of the latter group of females with methoprene, a JH analogue, restored production of apoVg-I whereas injection of 20-hydroxyecdysone (20-HE) did not. When pharate adults were kept at 10°C from 10–12 h before the time of their anticipated eclosion, eclosion was delayed for 4–7 d. In such females, maintained at the same temperature, apoVg-I appeared only on day 3, and its appearance could be accelerated by two days with methoprene treatment immediately after eclosion. Decapitation of chilled females just after eclosion prevented the appearance of apoVg-I even after 16 d at 10°C. Likewise, Vg was absent in chilled females decapitated after eclosión and then maintained at room temperature. Methoprene induced apoVg-I production one day after treatment in chilled decapitated females maintained after eclosion either chilled or at room temperature. 20-HE had no effect on these females. Methoprene did not induce Vg in adult males. ApoVg-I ofH. zeadid not cross-react withManduca sextaandPlodia interpunctellaVg antibodies. These results demonstrate that Vg synthesis inH. zeais regulated by JH.