Activity of the corpora allata (CA) in vitro of adult female Gryllus bimaculatus was studied following incorporation of radioactivity from [2- 14C]acetate and l-[methyl- 3H]methionine into juvenile hormone III (JH III) and its immediate precursor methyl farnesoate (MF). Spontaneously active glands from females reared at 27°C utilized exogenous labelled acetate extensively for synthesis of MF and JH III (incorporation 80–84% at 2 mM acetate). 10 −7 to 10 −5 M exogenous JH III in the incubation medium had no effect on the rate of JH biosynthesis in spontaneously active glands. At 10 −4 M JH III incorporation of acetate into JH III was reduced. The amount of MF was also lowered. JH III treatment (10 −8–10 −6 M) of spontaneously inactive glands led to an increase in the amount of MF. This increase was due to a de novo synthesis. Exogenous farnesol (20–200 μM) increased JH III biosynthesis and the amount of MF, but suppressed [2- 14C]acetate incorporation. Dilution of the endogenous precursors is probably the most important cause of this suppression. As shown by the abnormally high MF levels in farnesol treated glands, epoxidation seems to be a rate-limiting step under certain experimental conditions.