The precocious induction in vivo and in culture of insect and amphibian metamorphosis by exogenous ecdysteroids and thyroid hormones, and its retardation or inhibition by juvenile hormone and prolactin, respectively, has allowed the analysis of such diverse processes of post-embryonic development as morphogenesis, tissue remodelling, functional reorganization, and programmed cell death. Metamorphosis in vertebrates also shares many similarities with mammalian development in the late foetal and perinatal period. This review describes the regulation of expression of some of the 'adult' gene products during metamorphosis in invertebrates and vertebrates. Recent studies on metamorphosis have revealed the important role played by auto-induction of hormone receptor genes, based on which a model will be presented to explain the activation of 'downstream' genes which give rise to the adult phenotype. It will also be argued that metamorphosis is an ideal model for analyzing some of the major mechanisms governing post-embryonic development.