A team from the University of Cincinnati, USA, has developed a novel glucagon receptor agonist that has enabled them to examine the action of glucagon. In mice, glucagon receptor agonism resulted in hyperglycaemia and reduced levels of body fat and plasma cholesterol. Activation of the glucagon receptor increased hepatocyte expression and circulating levels of fibroblast growth factor 21. These effects were confirmed in healthy human volunteers, in whom injection of glucagon increased plasma levels of fibroblast growth factor 21.