We examined the effects of a paramagnetic contrast agent, gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) on magnetic resonance images of acute myocardial infarction with and without reperfusion. Twenty-two dogs underwent occlusion of the left anterior descending coronary artery (LAD). In 10 dogs (group I) the LAD remained occluded for 3 hr and in the other 12 (group II) for 2 hr followed by 1 hr of reperfusion. Gd-DTPA (0. 34 mM/kg) was administered to five dogs in group I at 2 hr and 5 min after occlusion and to seven dogs in group II 5 min after reperfusion. At 3 hr after ligation, the hearts were excised and imaged with spin echo and inversion recovery pulse sequences on a 0.35 Tesla magnetic resonance imager. Reperfused hearts given Gd-DTPA demonstrated a significant increase in contrast between normal and reperfused myocardium as compared with nonreperfused hearts and reperfused hearts not given Gd-DTPA. This enhancement was particularly prominent in the inversion recovery images. Studies performed in vivo in two additional dogs demonstrated similar enhancement with reperfusion with Gd-DTPA in gated spin echo images. Contrastenhanced magnetic resonance imaging allows the detection of reperfusion early in the course of acute infarction.