In both Manduca sexta and Drosophila melanogaster, metamorphic events are driven by ecdysteroids whose production in prothoracic gland (PGs) is stimulated periodically by neural factors. Differences in the life cycle of moths and flies have made it difficult to compare the regulation of ecdysteroid biosynthesis in these two species. As in Manduca, at least two neural factors in the larval Drosophila BVG complex were separable by molecular weight, and they stimulated increased ecdysteroid biosynthesis from the ring gland, a composite organ that includes PG cells. Drosophila neural extracts accelerated ecdysteroid biosynthesis in Manduca PGs and, conversely, partially purified Manduca PTTH preparations elevated ecdysteroid biosynthesis in Drosophila ring glands, suggesting that the two species may share structurally similar prothoracicotropic factors. Drosophila ring glands required the presence of calcium ions to respond to neural extracts, but the phosphodiesterase inhibitor MIX and cAMP analogues exerted little, if any, positive effect on production. Mean ecdysteroid production rates of BVG-ring gland complexes taken from Drosophila larvae during various phases of the wandering period were often submaximal and highly variable, suggesting that they fluctuate widely prior to pupariation. Based on available data in Drosophila and the Manduca model for the control of ecdysteroid biosynthesis, a developmental scheme for neuroendocrine control in Drosophila is proposed.