A pulse duplicator for the testing of prosthetic valves has been produced which accurately simulates physiological pressure and flow wave forms in the left ventricle and ascending aorta. The model consists of two components--the ventricle and the artificial systemic circulation. The ventricle is a collapsible bag which is externally pressurized and produces an accurate ventricular pressure-time history. The artificial circulation is externally pressurized and produces an accurate ventricular pressure-time history. The artificial circulation is a development of the lumped parameter model of Westerhof13 in which the physiological input impedance is modeled by a characteristic resistance, a capacitance, and a peripheral resistance connected in series. The model allows for a wide range of heart rates, systolic-diastolic ratios, mean pressures, flow rates, and fluid viscosity. A Fourier analysis of the model pressure and flow waves shows excellent quantitative agreement with physiological data, as does the vascular input impedance. The Oxford aortic heart valve exhibited a regurgitation of 1.9 per cent and no measureable pressure drop or power loss.