There is increasing evidence that mechano-electric feedback, defined as a change in mechanical state that precedes and alters transmembrane potential, operates in a wide variety of preparations and species including man. Load reduction is becoming a common therapeutic tool in a variety of clinical settings but the electrophysiological effects of these manoeuvres is not known. In this study the effect of changes in loading conditions on the time course of ventricular repolarisation were examined in the in situ heart in 13 pigs anaesthetised with halothane. Monophasic action potentials, electrocardiograms and segment length changes were recorded from the left ventricular epicardium using suction operated devices. Afterload was decreased by intravenous infusion of sodium nitroprusside, and increased by aortic cross clamping. Infusion of sodium nitroprusside resulted in a rise in action potential duration (measured at 70% repolarisation) in all 21 infusions (mean 3.4 ms), which was linearly related to the fall in systolic left ventricular pressure (r = 0.72, p less than 0.001) and the change in minimum systolic segment length (r = 0.46, p less than 0.05), but not to the change in maximum diastolic length (r = 0.33, NS). Aortic constriction, sufficient to elevate peak systolic left ventricular pressure back to the control level, restored the changes in action potential duration to normal. In addition, there were concomitant changes in the QT interval and T wave of the epicardial ECG. These findings show that mechano-electric feedback operates in the in situ heart and has potential importance in the clinical setting where changes in systemic blood pressure may directly alter cardiac electrophysiology.