The present study was carried out to investigate the effect of orotic acid on ischaemic/reperfused myocardial function and cardiac glycogen content in isolated working rat hearts. In a preliminary series of studies, hearts isolated from male Wistar rats (300-350 g) were perfused with oxygenated Krebs-Henseleit buffer containing cumulative concentrations of orotic acid from 0.01 to 10.00 mg l-1. In the concentration range of 0.01-0.10 mg l-1, orotic acid significantly improved left ventricular function. Therefore, in the second series of studies, rats were treated intravenously with 0.01 mg kg-1 orotic acid for 4 days. Hearts were then isolated and subjected to 30 min of global no-flow ischaemia followed by 10 min of reperfusion. Orotic acid treatment significantly improved post-ischaemic myocardial function and increased pre-ischaemic and post-ischaemic glycogen content of the heart. We conclude that orotic acid improves ischaemic/reperfused cardiac performance and this effect may be based on the elevation of myocardial glycogen content.