This study was designed to evaluate the potential application of MRI in differentiating static blood from thrombus, age-related changes of thrombus and the signal differences of the intravascular thrombus in various pulse sequences. External jugular vein was ligated at both upper and lower ends to form a static blood column, and thrombin was injected into the column to cause venous thrombosis in a total of 15 mongrel dogs. The MR images were obtained with T1-and T2-weighted spin echo and gradient echo techniques, immediately after the formation of static blood a nd after 2 hour, 1 day, 1 week, 2 weeks and 4 weeks of the formation of thrombus at a 2.0T MR unit. The signal intensities of the thrombus and adjacent muscles were compared stubjectively, and the signal intensity ratio was compared objectively by the measurement of the signal intensities using a cursor, The MRI findings were compared with histologic findings. The signal intensities of static blood were hyperintense in all pulse sequences, and those of 2-hour, 1-day and 1-week-old thrombi were hyperintense in all pulse sequences. The number of experimental thrombi which showed isointensity on T1-and T2-weighted image, and hypointensity on gradient echo image increased as thrombi aged. The signal intensities of 2-week-old thrombus were isointense on T1-weighted image, hyperintense on T2-weighted image, and hypointense on gradient echo imate, while those of 4-week-old thrombus were isointense on T1-weighted and T2-weighted image, and hypointense on gradient echo image in most experimental thrombi. There was a tendency to decrease in a signal intensity ratio as thrombi aged on T1 -weighted, T2-weighted and gradient echo images(p<0.01) Histologically, thrombus was not formed and lumen was filled with many red blood cells (RBCs) in 2-hour-old specimen, but fibrin mesh was visible and RBCs decreased in number in 1-day-old specimen, In 1-week-old specimen, vessel was contracted and lemen was filled with thombus, RBSs, platelets, many fibrins and capillary-like strucrutes. The histologic findings of 2 -week - old thrombus were similar to those of 1-week-old one except calcification. In 4-week-old specimen, vessels were contracted and lumen was obliterated with fibrosis and organization of the thrombus. Therefore, it is possible to diagnose thrombus, and to assess sequential changes of MRI findings of thrombus by using all pulse sequences, and these results can be essential bases for the interpretation of MR images of patients with venous thrombosis.