Quantitative evaluation of myocardial O2 balance can be accomplished by measurement of O2 supply, demand, and intracellular O2 concentration. Experimentally, these parameters are often related to coronary blood flow, cardiac contractility and mitochondrial NADH redox levels, respectively. Methods were developed to measure these 3 parameters in a local region on the myocardial surface in open-chest dogs. Local coronary blood supply was measured with the aid of a small surface thermistor, and cardiac work with a miniature strain gauge arch. NADH oxidation-reduction state was recorded using surface fluorometry through a fiber optic light guide. Transient anoxia produced by N2 breathing caused a rapid but reversible elevation in NADH levels, which was not always accompanied by a concomitant change in contractile force. Elevation of heart rate resulted in a rise in intramitochondrial NADH followed by an increase in coronary flow. In spite of the increased flow, NADH levels remained elevated, indicating a change in the O2 balance; this may indicate that autoregulation does not necessarily result in total compensation.