The daily rate of cell replication in the aortic endothelium of the normal rat falls from a maximum of 13% at birth to 0.1-0.3% at age 5-6 months. This residual rate of replication largely represents the turnover rate of the endothelium in normal animals. After renal hypertension of 2-3 weeks duration, however, the rate of replication rises 10-fold to an average value of 1.6%. This increase may represent an increase in turnover; however, it probably represent, at least in part, a proliferative response to cover the expanded luminal surface of the dilated vessel.