[eng] Levels of phosphorylated adenosine nucleotides, including the universal energy carrier adenosine 5'triphosphate (ATP) and its metabolites adenosine 5' diphosphate (ADP) and adenosine 5'monophosphate (AMP), define the energy state in living cells and are dependent mainly on mitochondrial function. In this article, we describe a method based on the luciferase-luciferin system used to measure mitochondrial ATP synthesis continuously in permeabilized mammalian cells and mitochondria isolated from animal tissues. We also describe a technique that uses the expression of recombinant targeted luciferase to report ATP content in different cell compartments. Finally, we describe an HPLC-based method for accurate measurement of ATP, ADP, and AMP in cultured cells and animal tissues. 2002 Elsevier Science (USA). All rights reserved.