Pacific bluefin tuna are active teleost fish with a large capacity for heat conservation and endothermy. They have a high metabolism, and hence the myocardium must be capable of sustaining elevated levels of cardiac output over a wide range of temperatures. To examine the way that the myocardial cells of bluefin tuna respond to their unique cardiac physiology, we have studied the ultrastructure of the internal membrane system and mitochondria of atrial and ventricular myocytes by light and electron microscopy. Our results reveal that cardiomyocytes of juvenile bluefin tuna posses a relatively high content of sarcoplasmic reticulum (SR), together with a large volume of mitochondria within the two (compact and spongy) ventricular compartments and in the atrial myocardium. The mitochondrial structure and distribution in bluefin tuna myocardium follow specific metabolic zonation resulting in a higher volume and lower cristae density in the compact ventricular layer than in atrium and spongy layer. The presence of junctional SR profiles and an extensive network of free SR within cells may ensure a rapid delivery of Ca(2+) to the myofibrils. This, in conjunction with transarcolemmal Ca(2+) entry, might contribute to a faster excitation-contraction-relaxation cycle and thus enhance cardiac performance, cardiac output, and the maintenance of excitability at low temperatures. We propose that the mitochondrial configuration together with the developed SR ultrastructure of bluefin tunas myocardium are important evolutionary steps for the maintenance of high heart rates and endothermy in this teleost fish.