The effect of ischaemia, either alone or after a single or multidose infusion of St Thomas' Hospital cardioplegia solution (ST), on endothelial function was studied by examining its influence on 5-Hydroxytryptamine- (5-HT-) and nitroglycerine- (GTN-) induced coronary vasodilation in the isolated rat heart. Eighty rat hearts were perfused on a modified Langendorff preparation. Thirty minutes of unprotected ischaemia (N = 16) abolished the vasodilatory response to 5-HT but maintained the response to GTN (expressed as % of the control value 5-HT, pre 45.0 +/- 3.5, post 2.3 +/- 3.2, GTN, pre 42.3 +/- 4.4, post 39.8 +/- 4.0). The increase in coronary flow induced by either agent was unaltered following 30 or 60 min of ischaemia protected by a single dose of ST (N = 16 in each). Ninety minutes of ischaemia preceded by a single infusion of ST (N = 16) markedly reduced the 5-HT response but maintained the vasodilation induced by GTN (5-HT, pre 37 +/- 2.6, post 6.9 +/- 2.6, GTN, pre 40.1 +/- 2.5, post 37 +/- 2.8). During 90 min of ischaemia, infusion of an additional dose of ST after 45 min (N = 8) or two extra doses every 30 min (N = 8) maintained the 5-HT-induced increase in coronary flow (pre 39.5 +/- 3.9, post 36.4 +/- 3.5). It can be concluded that coronary vascular endothelium is more susceptible to ischaemic damage when compared with vascular smooth muscle. This results in early loss of endothelium-dependent vasodilation. Multidose infusion of St Thomas' Hospital cardioplegia solution is superior to a single infusion in protecting endothelial function following prolonged cardiac ischaemia.