An antiallatal agent, precocene II. was used to assess the effect of the corpus allatum on vitellogenin synthesis/release and oöcyte development of the adult black blowfly, Phormia regina. Oöcyte development was greatly retarded by a treatment of precocene II given topically within 12 h after emergence. A single dose of 20 or 40 μg of precocene II per fly inhibited oöcyte development whereas a treatment given to flies more than 12 h after emergence was less effective. The precocene-inhibited oöcyte development could be reversed by a single treatment of methoprene—a juvenile hormone analogue. Scanning electron microscopy showed that the volume of the corpus allatum of precocene-treated females was significantly smaller than that of untreated females. Radiochemical assay showed that the juvenile hormone biosynthesis/release ( in vitro) by the corpus allatum of precocene-treated flies was significantly reduced but not stopped. In precocene-treated females, haemolymph protein and vitellogenin accumulated after a liver-meal, while ovarian soluble protein and vitellin content remained low. Application of methoprene lowered both the amount of haemolymph protein and vitellogenin and increased the quantities of the ovarian soluble protein and vitellin. Our data demonstrate that the major function of the hormone is to regulate the uptake of vitellogenin by developing oöcytes.