A radioimmunoassay was developed using 125I-labeled-[TyrO]Locusta-DH and polyclonal antibodies raised against Locusta-DH (29-46). The assay had a detection limit of 50 pM, and displayed limited cross-reactivity for other CRF-related peptides, but not for unrelated peptides. About 60% of the total immunoreactive material in locust hemolymph was attributable to Locusta-DH. The circulating level of diuretic hormone increases fivefold in fed insects, sufficient to stimulate primary urine production, and is correlated with the duration of the meal. This is consistent with the role of Locusta-DH in the control of postfeeding diuresis.