Blood was collected simultaneously from the left ventricle and pulmonary artery in 12 patients undergoing routine cardiac catheterisation and was analysed for noradrenaline concentrations at rest, during, and after isometric stress (hand grip). Moderate isometric exercise resulted in a significant rise in plasma noradrenaline with a return to basal values 10 minutes after discontinuing the grip test. There were no significant differences in noradrenaline levels between the left ventricular and pulmonary arterial samples either at rest or during exercise. Three patients with evident left ventricular dysfunction had the highest plasma noradrenaline concentrations, in contrast to the much lower levels in 2 patients on beta-blockers and in 1 patient with a normal heart. As moderate isometric effort results in an important increase in noradrenaline level, this form of exercise could be dangerous in subjects suffering from ischaemic heart disease or in those with impaired left ventricular function since these patients are particularly susceptible to arrhythmias.