The noninvasive measurement of systolic time intervals (STI) during surgery is expected to provide a useful index of the status of the patient's heart. A PDP-11 based system has heen developed to continuously measure and report these intervals in real time. The software has heen designed to allow easy implementation of a variety of monitoring tasks, in addition to STI. To accomplish this, a general purpose control program was developed which maintains a multipatient environment and performs all operations which are task independent. Applications modules, specific to the parameters being measured, are called from the control program. For STI, these modules operate on samples of the KG, carotid pulse, and heart sounds to obtain the onsets of the QRS, systolic upstroke, dicrotic notch, and second heart sound. Every 15 set, other modules derive mean and standard deviation of the preejection period (PEP), left-ventricular ejection time (LVET), and heart rate. Trend graphs of these parameters are displayed in the operating room. The accuracy of computer-determined ST1 was evaluated by comparing manual and machine measurements of 250 randomly selected heartbeats. The mean difference between the measurements was less than 1.5 + 7 (SD) msec.