The use of radioimmune assays to determine ecdysteroid titres during insect development has become widespread in recent years. In this review I consider the application of these assays to studies on the development of Drosophila melanogaster. In all, 8 studies have been undertaken with somewhat conflicting results. I discuss the underlying problems in these studies and relate them to studies in Calliphora and Manduca. I also consider evidence from in vitro studies of Drosophila tissues regarding the levels of circulating, biologically active, ecdysteroids and argue that the fluctuations so far described are not sufficient to explain the control functions ascribed to ecdysteroids in many developmental processes.