The anterior pituitary regulates the function of multiple organ systems as well as body growth, and in turn is controlled by peptides released by the hypothalamus. We find that mutation of the Gsh-1 homeobox gene results in pleiotropic effects on pituitary development and function. Homozygous mutants exhibit extreme dwarfism, sexual infantilism and significant perinatal mortality. The mutant pituitary is small in size and hypocellular, with severely reduced numbers of growth hormone- and prolactin-producing cells. Moreover, the pituitary content of a subset of pituitary hormones, including growth hormone, prolactin and luteinizing hormone, is significantly decreased. The hypothalamus, although morphologically normal, is also perturbed in mutants. The gsh-1 gene is shown to be essential for growth hormone-releasing hormone (GHRH) gene expression in the arcuate nucleus of the hypothalamus. Further, sequence and electrophoretic mobility shift data suggest the Gsh-1 and GHRH genes as potential targets regulated by the Gsh-1-encoded protein. The mutant phenotype indicates a critical role for Gsh-1 in the genetic hierarchy of the formation and function of the hypothalamic-pituitary axis.