Injection of β-ecdysone into the abdomen of diapausing prepupae of Monema flavescens induced an extra ecdysis, resulting in prepupal-pupal intermediates instead of normal pupae. If diapausing prepupae were allatectomized, diapause soon broke and pupation took place frequently. Implantation of corpora allata from diapausing donors into early post-diapause or allatectomized prepupae induced diapause again in the hosts. Hemolymph levels of juvenile hormone rose during early diapause period and fell gradually toward the end of diapause. Changes in volume of corpora allata occurred in parallel to rise and fall in hemolymph level of juvenile hormone. Brains from diapausing prepupae implanted into decapitated diapausing prepupae caused pupation in the hosts within a short period. By contrast, implantation of brain-corpora allata complexes from diapausing prepupae did not result in quick pupation of the recipients. Implantation of corpora allata into diapausing prepupae gave rise to a significant decrease in nuclear volume in neurosecretory B cells in the pars intercerebralis, while extirpation of corpora allata resulted in a significant increase. The increase in nuclear volume following allatectomy was suppressed by implantation of corpora allata from diapausing donors. Prepupal diapause in Monema flavescens seems to be initiated and maintained by an inhibitory action of juvenile hormone on the release of neurosecretory materials from B cells of the pars intercerebralis. When the period of active secretion of corpora allata comes to an end, the brain-prothoracic gland system is activated and diapause is broken.