Early trypsin is a female-specific protease present in the Aedes aegypti midgut during the first few hours after ingestion of a blood meal. The enzymatic activity of early trypsin plays an essential role in the transcriptional activation of the late trypsin gene, which encodes the major midgut endoprotease involved in blood meal protein digestion. Transcription of the early trypsin gene is part of the normal post-emergence maturation of the midgut in the adult female. Abdominal ligation within 1 h of emergence completely prevented the transcription of the early trypsin gene. Topically applied JH III or methoprene induced transcription of the early trypsin gene in ligated abdomens to levels similar to those observed in non-ligated females. The induction of early trypsin transcription by JH is dose-dependent and 'head-independent', suggesting that factors coming from the neuro-secretory axis are not required.