Alterations in hypothalamus-pituitary-adrenal (HPA) axis activity have been linked to the development of the metabolic syndrome (MetS). Common features of the MetS, like insulin resistance and obesity, are reproducibly induced by high fat diet (HFD) in animal models of diet-induced obesity. These models, hampered by methodological differences, reveal conflicting results with respect to HPA axis activation. This study was aimed to evaluate in detail nonstressed diurnal HPA axis activity in mice during obesity development. Male C57Bl/6J mice were fed high or low fat diet for 12 weeks. HPA axis activity was evaluated by plasma corticosterone concentrations (at 0700, 1200, and 1800 h), corticotropin-releasing hormone (CRH), and glucocorticoid receptor (GR) mRNA expression in the hippocampus, amygdala, and hypothalamus, and 11β-hydroxysteroid dehydrogenase type-1 and -2 (11β-HSD-1 and -2) expression in adipose tissue and liver. Within 1 week, the HFD induced obesity and decreased corticosterone levels at 1200 and 1800 h, which persisted throughout the experiment. Twelve weeks of HFD decreased CRH mRNA in the paraventricular nucleus (PVN) and amygdala and GR mRNA in the PVN at 0900 h. At 1800 h, CRH mRNA expression increased in the PVN and amygdala, and GR mRNA increased in the CA1 region. 11β-HSD-1 expressions decreased in gonadal, visceral, and subcutaneous adipose tissues at 0900 and 1800 h, whereas hepatic 11β-HSD-1 expression increased at 1800 h, whereas 11β-HSD-2 expression was unaffected. The HFD induces complex changes in the diurnal regulation of the different components of the HPA axis. These changes are not unequivocally characterized by increased, but rather by decreased HPA axis activity.