I. INTRODUCTION. The aortic valves are not opened at the very beginning of ventricular systole. The intraventricular pressure must be raised above the pressure in the aorta before the valves are opened, and in order to do this a definite, measurable length of time is required. This time is known as the presphygmic period or interval of the cardiac cycle. The purpose of the present work has been to measure the time of this period between the beginning of ventricular contraction and the opening of the aortic valves, and to study its variations under different pathological conditions. It seemed likely that the heart would be unable to overcome the aortic pressure and so send forth the pulse wave as quickly under certain abnormal as under normal conditions, and the study was undertaken with the hope of obtaining results that would be of practical clinical value, as well as an addition to