SYNOPSIS. Neuropeptides of the insect brain that regulate juvenile hormone synthesis by the corpora allata include allatotropins, stimulatory modulators, and allatostatins, inhibitory modulators. A radiochemical assay for juvenile hormone synthesis by corpora allata in vitro was utilized in the high pressure liquid chromatographic isolation of brain neuropeptides leading to the determination of their primary structure. Identified are an allatotropin and an allatostatin from a Lepidopteran, Manduca sexta, and a family of five allatostatins from a Dictyopteran, Diploptera punctata. These neuropeptides are all unique, effective at low concentration (10−10 to 10−8 M), act quickly (within hrs) and appear to be effective only within the same order of insects as that from which the peptides were isolated. The physiological state of the corpora allata conditions the effectiveness of the allatostatins of D. punctata. These neuropeptide regulators of corpora allatal function may have multiple regulatory roles. This is indicated for D. punctata allatostatin I by specific receptors in brain and fat body as well as in corpora allatal membrane preparations, and also by immunocytochemical localization of allatostatin I in medial nerve cells that terminate within the brain as well as in the lateral neurosecretory cells that terminate on corpus allatum cells.