When an active JH analog (methoprene or pyriproxyfen) is administered to JHdeprived (precocene-treated) adult female locusts, there is a delay of about 24 h before vitellogenin (Vg), the product of a JH-dependent gene, is found i n the hemolymph. If a dose of JH Ill or methoprene, itself too low to induce any detectable Vg, is administered 12-48 h before the inducing dose of JH analog, however, the appearance of Vg after the inducing treatment is accelerated by about 12 h. This effect i s described as priming. It is proposed that priming involves the synthesis of a specific transcription factor for Vg and other JHdependent genes. A system suitable for testing this hypothesis is provided by transcription in locust fat body nuclear extracts, using G-free cassettes as reporter sequences. Protein extracted from fat body of JH-deficient (precocenetreated) locusts transcribes from the control non-specific AdML promoter but not from the promoters of the two JH target genes. Preparations from normal adult female fat body, or from that of locusts treated with precocene and later with a JH analog, on the other hand, transcribe from the promoters of the JHdependent Vg and jhp21 genes, as well as the AdML promoter. This demonstrates the JH-induced appearance of a specific transcription factor. The cell-free transcription system is suitable for further analysis of the molecular components of JH regulated transcription. o