A study has been made of Time—Position ultrasonic scanning as a means of obtaining measurements related to cardiac performance. The errors involved in determining these measurements have been estimated. It has been shown that the greatest accuracy can be achieved for quantities which depend on action rather than shape such as ejection fractions and rates of myocardial fibre shortening. Attention is drawn to the fact that the ultrasonic method, unlike angiography, in no way alters the quantities being measured.