It has been suggested that collagen loss and damage is responsible for the dysfunction seen in stunned myocardium. To test this hypothesis we compared collagen in canine hearts stunned by repeated occlusion with collagen in hearts stunned by a single occlusion. Regional contractile function was equally depressed in both groups: segment shortening at 1 hour after reperfusion averaged 37% ± 9% versus 32% ± 9% of preocclusion values in repeated and single occlusion models, respectively. Midmyocardial collagen content was not different in either single occlusion (10.5% ± 0.4%) or repeated occlusion models (9.5% ± 0.7%) when compared with nonischemic hearts (8.5% ± 0.8%). Collagen damage, which was revealed with polarized light microscopy, was seen in 5 of 6 dogs after repeated occlusion but was not apparent after a single occlusion. Thus although both models of stunned myocardium produce similar dysfunction, there was no apparent collagen loss. Furthermore, collagen damage was only seen after repeated occlusion. Therefore it appears unlikely that collagen damage is a primary mechanism of stunned myocardium.