Skeletal muscle cells have been established as significant producers of IL-6 during exercise. This IL-6 production is discussed as one possible mediator of the beneficial effects of physical activity on glucose and fatty acid metabolism. IL-6 itself could be the exercise-related factor which upregulates and maintains its own production. We investigated this hypothesis and the underlying molecular mechanism in cultured C2C12 cells. IL-6 led to a rapid and prolonged increase in IL-6 mRNA, which was also found in human myotubes. Since IL-6 has been shown to activate AMPK we studied if, in turn, activated AMPK induces IL-6 expression. Pharmacological activation of AMPK with AICAR upregulated IL-6 mRNA expression, which was blocked by knock-down of AMPK 1 and 2 using siRNA oligonucleotides. However, the effect of IL-6 was shown to be independent of AMPK, since the siRNA approach silencing the AMPK subunits did not reduce the upregulation of IL-6 induced by IL-6 stimulation. The self-stimulatory effect of IL-6 partly involves a Ca²⁺-dependent pathway: IL-6 increased intracellular Ca²⁺ and intracellular blockade of Ca²⁺ using a Ca²⁺-chelator reduced the IL-6-mediated increase in IL-6 mRNA levels. Moreover, inhibition of Ca²⁺/calmodulin-dependent kinase kinase with STO-609 or si-RNA approach decreased IL-6 mRNA levels of control and IL-6-stimulated cells. A major, STO-609-independent mechanism is the IL-6-mediated stabilization of its mRNA. The data suggest that IL-6 could act as autocrine factor upregulating its mRNA levels thereby supporting its function as exercise-activated factor in skeletal muscle cells.