1. The present study determined whether two dimensional guided M-mode echocardiography could assess left ventricular (LV) geometry and function following a moderate size myocardial infarction in the rat. 2. Myocardial infarction (MI) was induced by left coronary artery ligation and rats were studied 4 weeks later. Infarct rats showed increased LV internal diastolic diameter (7.33 +/- 0.8 vs 5.91 +/- 0.6 mm; P < 0.001), LV systolic diameter (3.73 +/- 1.2 vs 1.87 +/- 0.6 mm; P < 0.001) and thickening of the noninfarcted posterior wall compared with sham operated rats (1.81 +/- 0.2 vs 1.47 +/- 0.3; P < 0.001; n = 10/group; mean +/- s.d.). Systolic function was impaired in infarct rats who showed reduced fractional shortening (50 +/- 12 vs 68 +/- 9%; P < 0.001) and fractional area change (41 +/- 14 vs 78 +/- 5%; P < 0.001). Infarct size measured echocardiographically was comparable to that measured by quantitative histological examination (29 +/- 10 vs 28 +/- 5%; NS). 3. The present study indicates that postinfarction remodelling leading to LV cavity dilation, hypertrophy of surviving myocardium and impaired systolic function is apparent 4 weeks following moderate size MI in the rat. 4. Transthoracic echocardiography is a noninvasive technique that may be used to assess serially the efficacy of therapeutic interventions designed to prevent remodelling in moderate size MI in the rat.