Consistent evidence indicates that modest consumption of fish or fish oil supplementation lowers the incidence of cardiac death and possibly other cardiovascular events. Precise physiologic mechanisms accounting for such reduced clinical risk are not well understood. Animal-experimental studies, randomized trials, and large observational studies indicate that consumption of fish and marine n-3 polyunsaturated fatty acids (PUFAs) affects cardiac haemodynamics. Independent effects include lowering of systemic vascular resistance (reducing systolic blood pressure by approximately 3-5 mmHg and diastolic blood pressure by approximately 2-3 mmHg); reduction of resting heart rate (by approximately 3 bpm); and improvement of cardiac diastolic filling (by means of potential improvement in both active relaxation and passive compliance). Stroke volume increases, apparently a secondary effect of lower vascular resistance, slower heart rate, and improved filling; direct effects on resting contractility or cardiac output are not evident. These haemodynamic effects could in part account for clinical benefits of fish or fish oil intake, including lower risk of cardiac death and possibly ischaemic stroke, heart failure, cognitive decline, and nonfatal coronary events.