The relationship between the energy state and intracellular oxygen concentration was established with cardiac tissue, where the former could be monitored by 31P-NMR and the latter by optical method for myoglobin absorption. The ratio of creatinephosphate to inorganic phosphate, PCr/Pi, started to fall at higher intracellular oxygen concentration than oxygen consumption (10IM). The pH shift measured by frequency shift of Pi paralleled the increase of lactate-release, of which half maximum was approximately 3IM. The critical oxygen concentrations differed nearly 5-fold when determined by energy state and oxygen consumption.