Specific sections describe the intrinsic properties of the heart, myocardial oxygen uptake, and the effects on cardiac performance of circulating catecholamines, acidosis, hypoxia, and temperature. Stroke volume (SVH) is primarily regulated by preload of the heart and heart rate (fH) (Frank–Starling effect). Routine variations in cardiac performance are probably accommodated by the intrinsic ability to increase cardiac output [Formula: see text] in response to preload and to maintain [Formula: see text] constant over a range of "afterloads" (homeometric regulation). The sinoatrial pacemaker governs fH and its frequency is altered primarily by temperature and vagal inhibition, secondarily by circulating catecholamines or adrenergic innervation, and probably not at all by preload or afterload in teleosts. Venous blood provides an ample myocardial O2 supply in benthic fishes. In more active fish, a coronary circulation and myocardial myoglobin are also important in O2 delivery. It is suggested that circulating catecholamines protect and maintain cardiac performance during stressful situations (e.g., hypoxia and acidosis) rather than to increase SVHper se. Lastly, temperature adaptations involve changes in SVH, fH and [Formula: see text].