Background: An important role has been suggested for tissue-specific glucocorticoid metabolism in the development of obesity and its complications. 11ß hydroxysteroid dehydrogenase 1 (11ßHSD1) is an enzyme that catalyzes the interconversion of biologically inactive cortisone to active cortisol, thereby regulating its access to glucocorticoid receptors in target tissues. Indeed, an unfavorable metabolic outcome has been associated with increased 11ßHSD1 gene expression and activity in adipose tissue and liver in humans and rodents. Cortisol is an important regulator of phosphoenolpyruvate carboxykinase (PEPCK) a key enzyme in gluconeogenesis and lipid metabolism. In rodents, overexpression of PEPCK in adipose tissue leads to adiposity and increased fatty acid re-esterification. In human obesity, PEPCK has been positively associated with body fat, total cholesterol levels, and plasma triglycerides. However, few studies have addressed the putative reversibility of peripheral cortisol levels and disturbed fatty acid homeostasis that may accompany weight loss. The aim of this thesis was to investigate alterations in peripheral glucocorticoid metabolism in the context of obesity, and putative modulations of glucocorticoid metabolism in the context of weight changes in humans and rodents. Materials & Methods: 11ßHSD1 expression/activity in different adipose tissue depots and liver, the expression of genes involved in adipogenesis and fatty acid homeostasis, and serum levels of adipose tissue-derived adipokines were investigated in severely obese women before and after surgically induced weight loss. The same parameters were measured in female Sprague-Dawley rats fed on high-fat and control diets. Results: In severely obese women, 11ßHSD1 expression was higher in subcutaneous adipose tissue (SAT), while 11ßHSD1 activity and PEPCK expression were higher in the omental depot. In a multivariate analysis, SAT 11ßHSD1 activity was an independent predictor for central fat accumulation. Hepatic 11ßHSD1 activity and levels of intra-abdominal fat storage correlated negatively, while 11ßHSD1 correlated positively with PEPCK in adipose tissue and liver. Weight loss after gastric bypass surgery was followed by significant and metabolically beneficial reductions in subcutaneous 11ßHSD1 and leptin gene expression, as well as reduced circulating leptin and increased adiponectin levels. In contrast, PEPCK gene expression did not change with weight loss. In rats, a high-fat diet did not affect body weight, but was associated with increased serum leptin and decreased adiponectin levels. Short-term, high-fat diet feeding resulted in the up-regulation of SAT 11ßHSD1 expression, while chronic feeding led to its significant down-regulation (compared with the control diet and short-term, high-fat feeding). Interestingly, hepatic 11ßHSD1 expression was constantly downregulated in rats that were fed a high-fat diet. Conclusions: Severe obesity in women was accompanied by a metabolically adverse increase of 11ßHSD1 in adipose tissue, with a concomitant decrease in the liver. Subcutaneous 11ßHSD1 was an independent predictor for central fat accumulation. As weight loss was followed by significant down-regulation of subcutaneous 11ßHSD1, we suggest that up-regulation of this enzyme was a consequence, rather than a cause of obesity. In rodents, a high-fat diet induced dynamic changes in 11ßHSD1 in SAT and liver, both being down-regulated after chronic high-fat feeding without altered weight. In summary, weight changes and alterations in fat and liver glucocorticoid metabolism are closely linked. Moreover, a high-fat diet significantly influences 11ßHSD1 expression/activity in adipose tissue and liver without affecting body weight.