1. Radiochemical in vitro assays show that there are large and rapid changes in the spontaneous rate of juvenile hormone synthesis and release during the course of sexual maturation in the locust. 2. Parallel observations on glands incubated with farnesenic acid show that the rate-limiting step is always prior to the stage of esterification of farnesenic acid to the final intermediate, methyl farnesoate. 3. Corresponding changes in oocyte morphometrics do not reveal any clear correlation between endocrine activity of the corpus allatum and either the induction or maintenance of rapid vitellogenesis. 4. Peaks of synthetic activity in the corpus allatum correspond well with the onset of previtellogenic growth in the oocytes. 5. The data do not provide any evidence that ‘corpus allatum insufficiency’ is responsible for resorption of growing oocytes. 6. It is concluded that short-term in vitro radio-assays provide a valid method for estimating quantitatively the physiological activity of the corpus allatum.

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