Incubation of partially purified rat liver lipid methyltransferase with MgATP and the catalytic subunit of the cyclic AMP dependent protein kinase results in up to 4-fold activation of the methylation reaction. When (gamma-32p) MgATP is included in the assay mixture, the analysis of the phosphoprotein products by electrophoresis shows the incorporation of 32p into a single protein band of about 50K and pI 4.75. It is concluded that rat liver lipid methyltransferase can be converted from a low activity dephosphorylated form to a high activity phosphorylated form.