Left atrial and left ventricular volumes, regurgitant flows and systolic efficiencies have been calculated for dogs with and without mitral valve insufficiency, using a technic of electric analog simulation of radioisotope dilution data. Amounts of blood between the valve of the pulmonary artery and the left atrium were calculated by use of mean transit time and flow, and compared to those of the analog. It is concluded that the analog technic is practical and will be more useful when data is available on flow patterns in the pulmonary bed.