The actions of steroid hormones on vertebrate and invertebrate nervous systems include alterations in neuronal architecture, regulation of neuronal differentiation, and programmed cell death. In particular, central nervous system (CNS) metamorphosis in insects requires a precise pattern of exposure to the steroid molting hormone 20-hydroxyecdysone (ecdysterone). To test whether the effects of steroid hormones on the insect nervous system are due to changes in patterns of gene expression, we examined Drosophila mutants of the ecdysterone-regulated locus, the Broad Complex (BR-C). This report documents aspects of CNS reorganization which are dependent on BR-C function. During wild-type metamorphosis, CNS components undergo dramatic morphogenetic movements relative to each other and to the body wall. These movements, in particular, the separation of the subesophageal ganglion from the thoracic ganglion, the positioning of the developing visual system, and the fusion of right and left brain hemispheres, are deranged in BR-C mutants. In addition, a subset of mutants shows disorganization of optic lobe neuropil, both within and among optic lobe ganglia. Optic lobe disorganization is found in mutants of the br and l(1)2Bc complementation groups, but not in those of the rbp complementation group. This suggests that the three complementation groups of this complex locus represent distinct but overlapping functions necessary for normal CNS reorganization. This study demonstrates that ecdysterone-regulated gene expression is essential for CNS metamorphosis, illustrating the utility of Drosophila as a model system for investigating the genetic basis of steroid hormone action on the nervous system.