The major juvenile hormone (JH) homolog synthesized in vitro by the adult female Medfly (Ceratitis capitata) corpus allatum (CA) is JHB(3), with JH-III the minor homolog. Methyl-incorporation in vitro in post-eclosion virgin females is age-dependent. Basal activity occurs during the first four days post-eclosion and increases significantly thereafter, peaking at five days. Biosynthetic maturation of the mated female CA is delayed by one day and reduced considerably. The delayed response may be due to direct cerebral or neural inhibition. Synthetic Drosophila melanogaster sex peptide depresses JH biosynthesis by the Medfly female CA in vitro. Male-derived accessory gland peptides of the Medfly are transferred to the female during mating and a Medfly SP-analog may be responsible for down-regulation of JH synthesis by the CA in mated Medfly females. Mevinolin, an inhibitor of the mevalonate pathway, significantly reduces the biosynthesis of JHB(3), while farnesoic acid, a proximate precursor of JHIII, significantly stimulates the biosynthesis of both JHB(3) and JHIII in vitro.