We determined whether the cell permeable molecule AICAR, whose metabolite activates AMP-activated protein kinase (AMPK) in cells, affected glycogen metabolism in rat soleus muscle preparations in vitro. The basal and insulin-stimulated rates of radiochemical lactate formation, net lactate release and glycogen synthesis were determined. AICAR stimulated net lactate release (but not radiochemical lactate formation) only at a basal concentration of insulin. An increased rate of glycogenolysis was the likely cause of increased net lactate release as glycogen phosphorylase activity vas significantly increased by AICAR. AICAR-stimulated net lactate release and phosphorylase activity were potently inhibited by insulin.