AbstractAdult femaleDrosophila melanogastercorpus allatum (CA) synthesize JHB3from endogenous and exogenous precursors in vitro. We present evidence supporting the thesis that biosynthesis proceeds from precursor FA via initial epoxidation and terminal methylation on the basis of the following: (1) Methyl farnesoate is not epoxidized to JHIII or JHB3; (2) Authentic JHIII is not epoxidized to JHB3; and (3) FABE is markedly metabolized to JHB3. Cerebral allatostatic factors act at some stage subsequent to FA and this precursor is not normally rate‐limiting. Additionally, neural inhibition from the brain acts at some biosynthetic step prior to FA. © 1995 Wiley‐Liss,