Glucocorticoid action in the brain is mediated by the glucocorticoid receptor (GR) and the mineralocorticoid receptor, thereby affecting physiological processes such as neurogenesis, synaptic plasticity and neuroendocrine control. To examine GR function in the regulation of the hypothalamic-pituitary-adrenal axis, we generated GR mutant mice that are homozygous for a conditional GR allele and heterozygous for a BAC-derived transgene that expresses the Cre recombinase under control of the regulatory elements of the mouse calcium/calmodulin-dependent protein kinase II alpha (CaMKII) gene resulting in Cre-mediated recombination in the brain and pituitary. The GR mutants die about one week after birth and display a fulminant increase in plasma corticosterone as well as a severe histopathological phenotype. To assess in which time-frame targeting of the pituitary occurs during embryonic development, we used a transgenic line expressing an inducible CreER^T2 fusion protein under the control of the regulatory elements of the CaMKII gene. Cre reporter data show that pituitary targeting occurred during embryonic development at the time when glucocorticoid synthesis starts.