BACKGROUND Left ventricular aneurysm formation after myocardial infarction (MI) has been associated with elongation of infarcted tissue in response to wall stress. Such elongation most commonly occurs in acutely infarcted or partially healed regions during the early post-MI period; however, recent reports have indicated that mature (15-week-old) healed infarct regions also undergo elongation after stress. RESULTS To assess factors contributing to post-MI left ventricular aneurysm formation, we subjected isolated strips (n = 50) of rabbit myocardial tissue from acutely ischemic (noninfarcted left ventricular), acutely infarcted (24 hours after MI), and healed infarct (3 and 15 weeks after MI) regions to a range of loading conditions and measured the reversible and irreversible length changes that occurred. The isolated strips were repetitively stretched for 1 hour at 4 Hz to impose cyclical physiological peak and resting stresses of 2.0 and 0.2 g/mm2. During a second hour, either peak stress ("afterload") or resting stress ("preload") was tripled, and the increase in strip length (strain) was measured. During a third hour, peak and resting stresses were returned to the initial values to assess the reversibility of length changes occurring during increased load. Elongation was expressed as the increase in natural strain from the first hour. Increasing afterload caused similar irreversible length increases of 4-5%/hr in acutely infarcted and 3- and 15-week-old healed infarct strips; acutely ischemic tissue length increased by 7.4%/hr (p less than 0.05 versus acutely infarcted tissue and scars). Increasing preload in acutely ischemic and acutely infarcted tissue caused a reversible length increase of less than 1%/hr. (Scar strips were not tested for the effect of preload.) CONCLUSIONS Since an irreversible length increase may represent an early event in aneurysm formation, our results suggest that 1) afterload increases are more likely to lead to aneurysm development than preload increases, 2) acutely ischemic tissue is the most vulnerable to increased afterload, and 3) for a given wall stress level, healing scar tissue is as susceptible to irreversible length changes as is acutely infarcted tissue. The observation that even mature post-MI scar elongated in response to increases in afterload implies that long-term pharmacological management of afterload in post-MI patients may be beneficial in preventing tissue elongation and aneurysm formation and that factors that increase wall stress (e.g., hypertension and exercise stress) have the potential to promote aneurysm formation in healed infarct scars.