AbstractIn the analysis of insect juvenile hormone (JH) function, metamorphosis is complicated by ecdysone and JH acting concurrently. Although JH stimulation of vitellogenin synthesis by the fat body is by no means simple, it is possible to focus on the events leading to selective transcription of a single gene. The two‐striped grasshopperMelanoplus bivittatuswas used to study nuclear binding of JH during vitellogenesis and to investigate proteins binding JH that have been found in hemolymph and fat body. M.bivittatusreproduction is regulated by JH. The fat body nuclei increased in ploidy with time after eclosion, and, because the antiallatotropin precocene completely inhibited vitellogenin production, we assume that this grasshopper differs little fromLocusta migratoriain which JH stimulates transcription of the vitellogenin gene. Proteins that bind JH with high affinity were identified with the hydroxyapatite assay. Characterization of the JH‐binding proteins included ion exchange chromatography, gel filtration, and polyacrylamide gel electrophoresis. Four proteins of hemolymph and three proteins of fat body bound JH with high affinity, and binding was competitively inhibited by JHIII. Short‐term tissue culture was used to follow [3H]JH‐III uptake by fat body and to determine the intracellular distribution of the hormone. Proteins that bound JH were extracted from nuclei with KCI, and they increased in number from the time of adult emergence to the time of maximum vitellogenin secretion. The JH‐binding proteins from nuclei might be receptors that help regulate the selective expression of genes for vitellogenin and for other proteins indu