The endocrine changes that initiate the metamorphosis of insect caterpillars into pupae and, ultimately, into adult moths occur over a very short time period. Thus, it is not surprising that titres of the hormones that control this process seem to be regulated by changes in the rates of both biosynthesis and degradation. A reduction in the titre of juvenile hormone (JH) early in the last larval instar has been shown to initiate metamorphosis and lead to a cessation of feeding behaviour This reduction in JH is associated with a dramatic increase in the levels of a very active, specific enzyme that hydrolyses the chemically stable, conjugated methyl ester to the biologically inactive JH acid. If this juvenile hormone esterase (JHE) is inhibited, the in vivo JH titre remains high enough to keep the larva in the feeding stage, resulting in giant insects. Thus, if sufficient quantities of JHE were expressed in vivo at an early stage of development, the reduction in JH titre should cause the affected insect to stop feeding. Because of its low abundance, the purification of sufficient JHE from the blood of caterpillars was very difficult until the development of a highly efficient affinity purification system. This system produced enough JHE for the development of antibodies and for partial amino-acid sequencing of JHE from several species including the major insect pest, Heliothis virescens. Three similar clones thought to encode JHE were obtained from a complementary DNA library made from fat bodies of H. virescens. Here, we describe the expression of JHE in an in vitro baculovirus system which will provide enough enzyme for detailed biochemical and physiological studies. The results suggest that in vivo expression of JHE may help in the improvement of genetically engineered viral insecticides which work by reducing insect feeding.