The purpose of this study was to determine if exercise undertaken during the phase of incomplete healing after myocardial infarction influences scar formation. Eighteen ether-anesthetized rats underwent coronary artery occlusion (CAO) and were paired by matching ECG infarct size as assessed by QRS morphology. One member of each pair was randomized to a nonswimming group (NoS) or a graded swimming (S) protocol group (up to 40 minutes of swimming per day) beginning 7 days after CAO. Twenty-one days after CAO, rats were reanesthetized, hearts were excised and examined under magnification, and were then sectioned for histology. Transmural scar thickness (mm) measured on gross pathologic specimens was thinner in the S rats (1.0 +/- 0.2, p less than 0.05) than in the NoS rats 1.4 +/- 0.3, p less than 0.05), while noninfarcted septal wall thickness (mm) was similar in the two groups (2.2 +/- 0.1 versus 2.1 +/- 0.1, respectively). The thinnest portion of the scar in S rats measured only 0.6 +/- 0.2 mm compared to that of NoS rats (1.1 +/- 0.3 mm, p less than 0.05). In this experiment exercise during the healing phase of acute myocardial infarction (AMI) caused thinning of the transmural scar.