The recovery of any tissue following a period of ischaemia is dependent on a patent microvasculature to restore blood flow. In the ischaemic myocardium, a reduction in capillary cross-sectional dimensions occurs, which is likely to contribute to "no-reflow" injury. Clinical and experimental evidence indicates that the retina is able to tolerate moderate periods of ischaemia without significant loss of function. The aim of the present study is to test the hypothesis that, as an end-arterial system, the retina possesses compensatory processes to maintain a functional microcirculation following acute ischaemia. Thirty minutes of no-flow global ischaemia was induced in isolated hearts of Wistar rats without reperfusion. The retina was also made ischaemic for 30 min using two experimental models: microsphere embolization and anoxic superfusion. Changes in capillary dimensions were assessed by ultrastructural morphometry. Following 30 min of myocardial ischaemia capillaries appeared swollen with a significant reduction in total capillary and luminal cross-sectional area. By contrast, ischaemic retinal capillaries showed minimal morphological changes and no significant alteration in dimensions. We have demonstrated notable differences in the response of retinal and myocardial microvessels to acute ischaemia. It is likely that the maintenance of capillary patency following short periods ischaemia in the retina is part of an adaptive mechanism to protect visual function.