Juvenile hormone esterase (JHE) activity released by the corpora allata (CA) into incubation media (CA-JHE) was titered daily during the course of the last (fifth [V]) larval stadium of Manduca sexta. This CA-JHE activity was relatively low during the early last stadium up to the time of commitment (V4), then rose rapidly to a peak on V6. Activity declined sharply almost to precommitment levels by V8, before rising to a second peak o n the first day of the pupal phase (PO). This pattern of activity i s distinct from that of hemolymph JHE activity, which peaks just prior to wandering o n V4 and again just prior to pupation (V9). Although the CA-JHE and hemolymph-JHE possess different temporal patterns of activity, isoelectric focusing, gel electrophoresis, and initial studies with selected inhibitors suggest that the enzymes responsible for the CA-JHE and hemolymph-JHE activities are similar, but not identical, in nature.Exposure of the V6 CA in vitro to JH II (0.1 pM) or fluoromevalonolactone (FMev; 0.1 mM) produced an approximate fivefold increase and 60% decrease in JH acid synthesis, respectively. Conversely, the same treatments resulted in an inhibition (JH II) and stimulation (FMev) of CA-JHE activity. These observations suggest that JH may be involved in the direct positive feedback regulation of postwandering larval CA and that the CA-JHE may also be integrally related to this positive feedback mechanism.Key words: endocrine feedback, hemolymph JH esterase, fluoromevalonolactone Acknowledgments: The authors thank Louise Studley for rearing the insects, Charles Allen and William Janzen for their help with the dissections, and Jay P. Woodring, Abner Hammond, Tad Hardy, and R. Mike Roe for their very useful discussions and manuscript comments.