Genetic lines of the cricket Gryllus assirnilis selected for elevated vs. decreased activity of hemolymph juvenile hormone esterase (JHE) exhibited a 6.8-fold difference in enzyme activity by the seventh generation of selection. This documents that hernolymph JHE activity has the capability for rapid evolutionary change. This is the only insect endocrine trait for which such data are currently available. The difference in heniolyrnph JHE activity between the lines was due, to an equivalent degree, to variation in whole-cricket enzyme activity and allocation of JHE activity to the hemolymph compartment. N o differences in kinetic or thermostability characteristics were observed between JHEs from the high-vs. low-selected lines. This suggests that the variation in JHE activity between the lines resulted from selection of variable regulatory genetic factors affecting the synthesis, degradation, or activation of enzyme activity. JHE and JH-binding activity in the hemolymph are genetically correlated, possibly due to co-regulation by the same genetic factors. Finally, in vivo JH degradation was significantly elevated in lines selected for elevated ]HE activity. This documents that evolutionary changes in JHE activity alter in vivo JH metabolism.