The film demonstrates two methods of estimating the cardiac output in man. Animated diagrams illustrate the earlier efforts of Stephen Hales, and also explains the Fick principle of estimating cardiac output. Measurement of oxygen consumption by spirometry is followed by technical details of cardiac catheterisation and sampling of blood from the right heart and the brachial artery. The analysis of samples on a Haldane blood gas apparatus is shown with diagrammatic explanations. Optical pressure recording from the right heart by means of a strain gauge is illustrated. The Hamilton dye method of estimating cardiac output is then shown. The principles involved are illustrated in animated diagrams, and the actual technique of dye injection and collection of arterial samples is seen. The results of the two methods of estimating cardiac output are compared showing reasonably good agreement of the two techniques. Features Werner Forssman describing his method of catheterisation, developed in 1929.