Acetyl-CoA carboxylase (ACC) can be regulated in vitro via phosphorylation by a 5'-AMP-activated protein kinase. A potential intracellular role for this kinase has been studied in the Fao hepatoma cell by manipulating the intracellular adenine nucleotide pool with ATP-depleting agents. Three different ATP depletors, antimycin A, dinitrophenol, and sodium azide, all promote the rapid loss of ACC activity characterized by a marked reduction in enzyme Vmax, abolition of citrate-independent activity, an increase in the Ka for citrate and a reduction in the mass of a complex between the two major ACC isozymes. These effects persist through enzyme purification on monomeric avidin-Sepharose and are accompanied by an increase in 32P-content, both consistent with depletor-induced covalent enzyme modification. The effects of ATP depletors in intact cells are mimicked in vitro on phosphorylation of ACC by the 5'-AMP-activated protein kinase and are reversible on dephosphorylation. These data indicate that ACC activity is sensitive to the intracellular adenylate charge, but that changes in the state of enzyme phosphorylation, rather than direct allosteric regulation by adenine nucleotides, underly this mode of enzyme control. This kinase-mediated modulation provides a mechanism for altering the rate of fatty acid synthesis and, secondarily, fatty acid oxidation, depending on the rate of ATP generation from carbohydrate-derived precursors in several tissues in vivo.