Telomerase activity is usually undetectable in adult human tissues, but is positive in embryonic tissues and in cancers. In rodents, however, several organs of normal adult animals express substantial amounts of telomerase activity. In this study, we observed the changes in the telomerase activity in rat organs during growth and aging and found that telomerase activity showed chronological patterns which were characteristic to organs. In lung and brain, the high telomerase activity of embryonic stage decreased rapidly after birth. In lymphoid tissues, telomerase activity increased after birth and reached to its maximum at 4 to 7 week. In liver, it stayed at nearly constant level throughout life. The telomerase activity in regenerating rat liver decreased temporally immediately after partial hepatectomy, then increased to a level that is higher than normal control. In contrast, it rapidly diminished in the occluded lobes after ligation of portal vein branch.