Thromboembolic stroke appears to evolve in patients in a very complicated manner. The present study investigated the evolution of thromboembolic stroke in rats (n=9) using a 4.7-T MR imager. Under isoflurane anaesthesia, the rats received homologous blood clots into the right internal carotid artery. After thromboembolic stroke, lesion volume, which was defined and calculated, based on apparent diffusion coefficient maps, tended to increase gradually over the 6 h magnetic resonance imaging study. The largest percentage change in lesion volume was found at the early stage (40-100 min) of thromboembolism, and showed significant correlation with total percentage change in lesion volume (41.6 (SD 32.8%)) (r=0.77, P<0.05). In conclusion, marked enlargement or diminution of lesion volume may be observed at the early stage of thromboembolism. Thromboembolic stroke, which can be partly salvaged at the early stage, may likely evolve to a lesser extent thereafter.