Hydroxymethylglutaryl-CoA (HMG-CoA) reductase kinase has been previously assayed by its ability to inactivate HMG-CoA reductase in crude rat liver microsomes, using a radioisotopic assay for HMG-CoA reductase. However, this assay is insensitive and time consuming and it uses expensive radioisotopes and is subject to interference by HMG-CoA lyase and mevalonate kinase. This chapter presents a completely specific and more convenient assay involving phosphorylation of a synthetic peptide (SAMS peptide) with the sequence HMRSAMSGLHLVKRR. This peptide is based on the sequence from His-73 to Lys-85 in rat acetyl-CoA carboxylase, except that the fifth residue is alanine rather than serine, to abolish a site (corresponding to Set-77) phosphorylated by cyclic adenosine monophosphate (AMP)-dependent protein kinase. In addition, two arginines are added at the C terminus to make the peptide bind tightly to phosphocellulose paper. The AMP-activated protein kinase phosphorylates the peptide on the seventh residue, corresponding to Set-79 on acetyl-CoA carboxylase, the site which is phosphorylated most rapidly by the kinase and which appears to regulate acetyl-CoA carboxylase activity.