Changes in RNA during the juvenile hormone (JH) stimulated synthesis of vitellogenin (Vg) in the fat body of adult female Locusta migratoria have been studied. Total RNA from mature females, but not that from males or prereproductive females, shows a 6300 nucleotide component, which has been isolated by binding to oligo(dT)-cellulose and sucrose gradient centrifugation, and identified as Vg mRNA by translation in Xenopus oocytes. It has been assayed quantitatively by photometric scanning after electrophoresis. During a gonotrophic cycle, Vg mRNA increased rapidly from 0 up to about 1% of the total fat body RNA, or more than 106 copies per cell. After destruction of the corpora allata (the source of JH) by treatment with ethoxyprecocene, Vg synthesis was stimulated by injection of 150 μg of the JH analog, methoprene. In primary stimulation, Vg mRNA was first detected at 24 h and showed a marked lag in accumulation; in secondary stimulation by methoprene after decay of the primary effect, Vg mRNA was detected after only 12 h and accumulation was much more rapid. Both in the natural cycle and in experimental stimulation, Vg mRNA did not disappear in correlation with declining Vg synthesis, which suggests conservation of mRNA in untranslated form. Total (chiefly ribosomal) RNA showed a different pattern, accumulating markedly during primary and only slightly during secondary stimulation. The data indicate that JH acts selectively (though not necessarily directly) on transcription of the Vg genes.