The high count sensitivity of the non-imaging nuclear probe affords the possibility of measuring left ventricular ejection fraction continuously during short term interventions. The nuclear probe was used to examine the pattern of change of left ventricular function during dynamic exercise and its temporal relation to ST segment depression in 12 patients with stable exertional angina. After in vivo blood pool labelling with technetium-99m the left ventricular time-activity waveform was detected by the nuclear probe and was continuously recorded on a strip chart. The 15 beat mean ejection fraction and the ST segment level 80 ms after the J point were measured at rest and every 30 seconds during maximal ergometric exercise and during recovery. The mean ejection fraction was 54.3% (range 46-64%) at rest and fell during exercise in all subjects by a mean of 16.8% (range 6-25%). In contrast, in a control group of 16 healthy male volunteers the mean ejection fraction was 55.9% (range 47-64%) at rest and increased in all by a mean of 10.2% (range 3-19%) during exercise. The difference of ejection fraction response to exercise between the patients and controls was due to pronounced increases in relative end diastolic and especially end systolic volumes in the patients. Relative stroke volume differed between patients and controls only at peak exercise. ST segment depression greater than 1 mm developed in 11 of the 12 patients. A decrease of greater than 5% in ejection fraction occurred within 1 minute of starting exercise in nine of the 12, and in 11 patients it preceded the beginning of ST depression. In most of this selected group of patients the ejection fraction had fallen during exercise before the appearance of ischaemic electrocardiographic changes.