In response to exercise the heart increases its metabolic rate several fold while maintaining myocardial ATP concentration relatively constant, however the mechanisms responsible for the regulation of this response are unclear. Limited experimental studies show that the classic regulatory species NADH and NAD⁺ are also maintained with increased cardiac power generation, but current measurements lump the cytosol and mitochondria, and do not provide dynamic information during the immediate transition from low to high work states. In the present study, we modified our previously published computational model of cardiac metabolism by incorporating the parallel activation mechanism for ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and oxidative phosphorylation, and simulated the metabolic responses of the heart to an abrupt increase in energy expenditure. The model simulations showed that myocardial oxygen consumption, pyruvate oxidation, fatty acids oxidation and ATP generation were all increased with increased energy expenditure, while ATP and ADP remained constant. Both cytosolic and mitochondrial NADH/NAD⁺ increased during the first minutes (by 40% and 20%, respectively) and returned to the resting values by 10-15 mins. Furthermore, model simulations showed that alternative substrate selection (elevated arterial lactate or diabetic conditions) affected cytosolic NADH/NAD⁺, but had minimal effect on the mitochondrial NADH/NAD⁺, myocardial oxygen consumption, or ATP production. In conclusion, these results support the concept of parallel activation of metabolic processes with an abrupt increase in cardiac energy expenditure, and suggest that there is a transient increase in the NADH/NAD⁺ ratio that is independent of substrate supply.