The cardiovascular effects of an intravenous injection of soluble insulin and of acute hypoglycaemia were examined in six normal male subjects using multiple-gated radionuclide ventriculography. The basal left ventricular ejection fraction rose significantly from 47 +/- 3% (mean +/- SEM) to 54 +/- 3% p less than 0.01, within 5 min of the intravenous injection of insulin, and before any significant changes occurred in the blood glucose concentration. The ejection fraction subsequently rose to a peak of 72 +/- 5% coinciding with the onset of the acute hypoglycaemic reaction. This corresponded to the nadir of blood glucose and was associated with rises in heart rate, stroke volume and cardiac output. The heart rate returned to the resting value within 30 min of the acute hypoglycaemic reaction, but the ejection fraction, stroke volume and cardiac output were still elevated 90 min later. The peak ejection fraction value immediately preceded the maximal increment of plasma catecholamines released in response to hypoglycaemia. Thus, administration of intravenous insulin had a small, immediate, discernible effect on the cardiovascular system. A subsequent rise in left ventricular ejection fraction of much greater magnitude was stimulated by the development of acute hypoglycaemia, and was associated temporally with sympatho-adrenal activation. The use of radionuclide ventriculography showed that the haemodynamic changes provoked by hypoglycaemia produced a sustained effect on cardiac contractility.