Scans of the cardiac blood pool are of practical value and are being widely used. Simple methods of measuring cardiac output and blood volume are available, but they have not yet been exploited by clinicians. With the rapid advances in nuclear technology, practical and innocuous methods for measuring regional blood flow in the heart and other areas will probably become available in the near future. Such methods will undoubtedly be used by clinical investigators. Their widespread clinical application will depend on whether or not they provide a practical way of solving a recognized medical problem.