The identification of neuropeptides that inhibit juvenile hormone (JH) synthesis by the corpora allata (CA) has verified the existence of these allatostatins, which, from much experimental evidence, have long been postulated to occur. It also makes possible new approaches for studying the role of allatostatins in the regulation of JH synthesis. Allatostatins, localized immunocytochemically, occur in lateral neurosecretory cells of the brain that innervate the CA. Presumably their effect on the CA results from the release of allatostatins at these nerve endings. Allatostatins also occur in the hemolymph in cockroaches and have been shown to act o n the CA through this pathway. T h e ability of allatostatins to inhibit CA depends not only on the concentration of the peptides but also on the sensitivity of the CA to them. Male Diploptera punctata were treated with JH analog following denervation of CA and implanted with a previtellogenic or vitellogenic ovary or injected with saline. Animals implanted with a vitellogenic ovary, compared to the previtellogenic ovary or saline, showed significantly increased JH synthesis by their CA and a reduced amount of allatostatin in the hemolymph. The denervated CA from these JH analog treated animals, following implantation with a previtellogenic and vitellogenic ovary, showed a tendency toward increased and decreased sensitivity, respectively, to a given dose of allatostatin in vitro compared to those from saline injected controls. Experiments such as these suggest that changes in release of allatostatins and in sensitivity of CA to them could be postulated to be major factors regulating jH synthesis in the cockroach, o 1996 Wiiey-Liss, Inc.