The crustacean mandibular organ (MO) produces methyl farnesoate (MF), a juvenile hormone-related compound thought to have roles in crustacean reproduction and development. Therefore, the control of MF production by the MO has been of considerable interest. Current evidence indicates that the MO is negatively regulated by peptides present in the eyestalk (MO inhibiting factor, MO-IH). Several eyestalk neuropeptides have been identified that inhibit MF synthesis by MO incubated in vitro. The amino acid sequences of these MO-IH peptides are similar to peptides in the crustacean hyperglycemic hormone (CHH) family of neuropeptides. In addition, there appears to be a compound in the eyestalk that lowers hemolymph levels of MF in vivo but does not directly affect the MO in vitro. The inhibition of MF synthesis by eyestalk peptides involves the inhibition of farnesoic acid O-methyl transferase, the last enzyme in the MF biosynthetic pathway. The activity of this enzyme is affected by cyclic nucleotides, suggesting that these compounds may be involved in the signal transduction pathway mediating the effects of MO-IH.