A nonimaging, ECG-gated scintillation-probe system is described that permits real-time quantification, at high temporal resolution, of the time variation of left ventricular (LV) volume over a complete, average cardiac cycle. Linearity between counting rate and volume, probe positioning and background correction were investigated for cylindrically collimated (CC) and parallel-hole-collimated (PC) detectors. In 53 patient studies, results obtained with these probes were compared with results obtained from an ECG-gated .gamma. camera system (CS) with high temporal resolution. Time-activity curves obtained by all 3 devices were essentially identical in shape (for CC against CS, r = 0.93; for PC against CS, r = 0.98) and intracycle timing. LV ejection fractions obtained with the probes showed workable agreement with the camera: for CC against CS, r = 0.85 (n = 31); for PC against CS, r = 0.90 (n = 21). When LV background is removed as a source of error, the correlation between (PC) probe and camera is improved (r = 0.95, n = 21). The portable probe system should apparently be used in circumstances where exact knowledge of LV background is minimally important, e.g., continuous bedside monitoring of changes in LV function.