Recent studies have highlighted the importance of an inhibitory phosphorylation site, Ser^485/491, on the α-subunit of AMP-activated protein kinase (AMPK); however, little is known about the regulation of this site in liver and skeletal muscle. We examined whether the inhibitory effects of insulin on AMPK activity may be mediated through the phosphorylation of this inhibitory Ser^485/491 site in hepatocytes, myotubes and incubated skeletal muscle. HepG2 and C2C12 cells were stimulated with or without insulin for 15-min. Similarly, rat extensor digitorum longus (EDL) muscles were treated +/− insulin for 10-min. Insulin significantly increased Ser^485/491 p-AMPK under all conditions, resulting in a subsequent reduction in AMPK activity, ranging from 40% to 70%, despite no change in p-AMPK Thr¹⁷². Akt inhibition both attenuated the increase in Ser^485/491 p-AMPK caused by insulin, and prevented the decrease in AMPK activity. Similarly, the growth factor IGF-1 stimulated Ser^485/491 AMPK phosphorylation, and this too was blunted by inhibition of Akt. Inhibition of the mTOR pathway with rapamycin, however, had no effect on insulin-stimulated Ser^485/491 p-AMPK. These data suggest that insulin and IGF-1 diminish AMPK activity in hepatocytes and muscle, most likely through Akt activation and the inhibitory phosphorylation of Ser^485/491 on its α-subunit.