After transmural myocardial infarction (MI), extensive myocardial remodeling by fibrous tissue appears in both infarcted and noninfarcted myocardium, which contributes to ventricular diastolic dysfunction. In the present study we sought to assess the time course of collagen remodeling in the infarcted rat hearts by detecting spatial and time-dependent cellular events related to collagen synthesis and degradation 2 to 28 days after left coronary artery ligation. In infarcted hearts, and compared with findings in sham-operated and unoperated rat hearts, we found the following: (1) macrophages infiltrated into sites of MI and visceral pericardium on day 2 and gradually disappeared after day 14; (2) myofibroblasts (MyoFb) first appeared at these sites of repair on day 3 and remained abundant thereafter at all time points examined; (3) transforming growth factor-beta1 (TGF-beta1) mRNA was enhanced in infarcted and noninfarcted myocardium on day 2 and remained throughout 28 days; (4) type I and III collagen mRNAs began to increase at and remote to MI on day 3 and remained elevated thereafter; (5) matrix metalloproteinase-1 mRNA was significantly increased at and remote to MI on day 3, declined to the control level on day 7, and remained low thereafter; (6) tissue inhibitor of matrix metalloproteinase (TIMP)-I, -II, and -III mRNAs were markedly elevated at sites of repair on day 3 and sustained throughout 28 days; (7) fibrillar collagen accumulation that was evident at and remote to MI on day 7 continued to accumulate thereafter at each site over 4 weeks. When compared with findings in unoperated rat heart, pericardial fibrosis was evident in both infarcted and noninfarcted heart, and the temporal response of collagen generation/ degradation in pericardium was similar to that in infarcted myocardium. Thus collagen synthesis is activated in both infarcted and noninfarcted rat myocardium after transmural anterior infarction and is persistent throughout the 28-day period of study, whereas early collagen degradation is short lived and inactivated in the fibrogenic phase. Activated TGF-beta1 mRNA expression is accompanied by the appearance of MyoFb and the expression of fibrillar collagens and TIMPs, suggesting that this fibrogenic cytokine may contribute to collagen remodeling in the rat heart after MI.