This study aims to determine whether glucose intervenes in the regulation of lipid metabolism in long-term fasting birds, using the king penguin as an animal model. Changes in the plasma concentration of various metabolites and hormones, and in lipolytic fluxes as determined by continuous infusion of [2-³H]glycerol and [1-¹⁴C]palmitate, were examined in vivo before, during and after a 2 h glucose infusion under field conditions. All the birds were in the phase II fasting status (large fat stores, protein sparing) but differed by their metabolic and hormonal statuses, being either non-stressed (NSB; n = 5) or stressed (SB; n = 5). In both groups, glucose infusion at 5 mg.kg^-1.min ^-1 induced a 2-fold increase in glycemia. In NSB, glucose had no effect on lipolysis (maintenance of plasma concentrations and rates of appearance of glycerol and non-esterified fatty acids) and no effect on the plasma concentrations of triacylglycerols (TAG), glucagon, insulin or corticosterone. However, it limited fatty acid (FA) oxidation, as indicated by a 25% decrease in the plasma level of -hydroxybutyrate (-OHB). In SB, glucose infusion induced a ~ 2.5 fold decrease in lipolytic fluxes and a large decrease in FA oxidation, as reflected by a 64% decrease in the plasma concentration of -OHB. There were also a 35% decrease in plasma TAG, a 6.5- and 2.8-fold decrease in plasma glucagon and corticosterone, respectively, and a 3-fold increase in insulinemia. These data show that in fasting king penguins glucose regulates lipid metabolism (inhibition of lipolysis and/or of FA oxidation) and affects hormonal status differently in stressed vs non-stressed individuals. The results also suggest that in birds, as in humans, the availability of glucose, not of FA, is an important determinant of the substrate mix (glucose vs FA) that is oxidized for energy production.