A hydromechanical simulator of the human left heart and systemic circulation system has been built for evaluating the dynamic actions of mitral valve prostheses. The facility includes a transparent and compliant model ventricle pumping into a simulation of the systemic circulation impedance. Transparent materials permit detailed high speed photography of valve action from the left ventricular aspect. This action has been analyzed by digitizing the valve open area and computing the variation of area with time. Additionally, pressures across and flows through the valves have been measured. Comparison has been made between the maximum observed valve open areas and the areas computed using the appropriate pressure and flow measurements substituted in clinically used equations.