In Drosophila, the ecdysteroids trigger the key regulatory cascades controlling the coordinated changes in the developmental pathway of molting and metamorphosis. Ecdysone action is mediated by a heterodimer consisting of the three ecdysone receptor (EcR) isoforms and the ultraspiracle proteins (USP). Heterodimers of these proteins bind to the ecdysone response element and ecdysone to modulate gene transcription. In this study, we developed a competitive reverse transcription polymerase chain reaction (RT-PCR) method to quantify the transcripts of functional ecdysone receptor components in individual tissues and for the whole body. The relatively small amount of variation in usp transcripts of the different tissues indicates that this gene does not perform a spatially restricted function in the late third instar wandering larvae while EcR isoforms were expressed in a more tissue-restricted pattern in the same stage. EcR-B1 was expressed at higher levels in larval tissues that are fated for histolysis, whereas EcR-A predominates in the imaginal discs. This result supports the hypothesis that a particular metamorphic response requires particular EcR isoforms. The transcript levels of the functional ecdysone receptor components fluctuate dramatically during development, suggesting that the regulations of the transcriptional and posttranscriptional levels of these genes play some role in ecdysteroid response during Drosophila development.