The most recent anatomical, neurochemical and experimental data on the hypothalamo-hypophysial system in vertebrates suggest that (a) the basic mechanism of regulation dates back very early in vertebrate phylogeny; (b) the fundamental plan of organization for tetrapods is present in amphibians; (c) the interactions between neurohormones and neurotransmitters in neuroendocrine regulation are extremely complex; and (d) a comprehensive explanation of the evolutionary role of neuroendocrine regulation is still lacking.