The normal range of aortic blood velocity was established in 140 healthy adults, using a non-invasive Doppler ultrasound technique. Integration of the area under the velocity-time curve for each heart beat gave stroke distance, which, when multiplied by heart rate, gave minute distance. Stroke distance and minute distance are an indication of stroke volume and cardiac output respectively and both show a progressive decline with age of about 1% per annum of adult life. Stroke distance gave the greatest discrimination between patient groups and, compared with age corrected normal values, was increased by 43% in 12 pregnant women, normal in 16 patients convalescing after myocardial infarction, and decreased by 14% in 15 patients with hypertension, by 31% in 12 patients with atrial fibrillation, and by 43% in 13 patients with cardiac failure. Measurement of aortic blood velocity and its derivatives alone, without determination of aortic size, is a safe, simple, and physiologically valid way of assessing cardiac output.