Juvenile hormone (JH) plays an important role in insect development and reproduction. Molting and metamorphosis are regulated by both JH and 20-hydroxyecdysone (20E). While 20E initiates the developmental transition from larva to pupa, JH maintains juvenile status and prevents this transition. 20E molecular action has been well studied, however, JH regulatory mechanisms and JH-20E interactions remain unclear. JH could regulate gene expression directly, or modify the expression of genes activated by 20E, such as BRC (Broad complex). Methoprene, a JH analogue is used to control mosquitoes. Methoprene-treated mosquitoes die during metamorphosis. To understand the molecular mechanisms of JH and methoprene action, we have determined JH III levels and the mRNA developmental profiles of several JH-inducible genes during last larval and pupal stages of Aedes aegypti. Juvenile hormone III is the major JH detected during the last larval stage. Low levels of JH III were present in the early stages of the fourth instar, JH III levels gradually increased and reached maximum levels by 30-33 hr after molting into the fourth instar. Then the JH III levels sharply decreased and reached undetectable levels by the end of the fourth instar. Quantitative real time PCR determination of mRNA levels of several JH inducible proteins (JHIP) showed that the mRNA levels of some of the JHIPs correlate well with the JH III levels while others do not. These initial studies provide tools to study mosquito development and the molecular basis of JH action.