During adult metamorphosis, the moth olfactory neurons and their glia-like support cells pass through a coordinated and synchronous development. By 60% of development, the olfactory system is anatomically complete, but functional maturation does not occur until about 90% of development. Maturation is characterized by the onset of odorant sensitivity in the sensory neurons and the expression of certain antennal-specific proteins including odorant binding proteins (OBPs) and odorant degrading enzymes (ODEs). The OBPs have been cloned and sequenced, and are thus useful models for investigating the molecular mechanisms coordinating final maturation of the developing olfactory system. The ecdysteroid hormones have been observed to regulate many cellular level neuronal changes during adult metamorphosis. In particular, the late pupal decline in ecdysteroids is known to influence programmed death of nerves and muscles at the end of metamorphosis. Experiments are presented here which indicate that this decline in ecdysteroids also induces the expression of the OBPs. Normal OBP expression occurs 35-40 h before adult emergence. In culture, OBP expression could be induced at least 90 h before adult emergence by the premature removal of ecdysteroid. This premature expression was blocked by culturing tissue in the presence of the biologically active ecdysteroid 20-hydroxyecdysone. These findings suggest that maturation of the olfactory system is regulated by the decline in ecdysteroids, and support the view that olfactory development, in general, may be coordinated by changing levels of pupal ecdysteroids.