1. Plasma renin concentration (PRC) and blood pressure were studied sequentially, 24 h to 42 days post-operatively, in rats subjected to unilateral renal artery constriction without contralateral nephrectomy. 2. The PRC of rats failing to develop hypertension remained normal, whereas the mean PRC of twenty-two rats that became hypertensive was five times normal on day 14 of the study when the hypertension was becoming established. 3. In eleven of the twenty-two rats that became hypertensive, PRC did not exceed the upper limit of normal. In the remaining hypertensive rats, the increase in PRC was not always temporally related to the increase in blood pressure. A significant correlation between PRC and blood pressure did not emerge until day 35 of the study. 4. Despite these anomalies, linear regression analysis of 169 pairs of PRC and blood pressure measurements during the 42 day period of development of hypertension in twenty-two rats revealed a highly significant correlation between log PRC and blood pressure (P smaller than 0.001). 5. It is concluded that factors other than the plasma concentration of renin are involved in the early stages of development of hypertension induced by renal artery constriction. Nevertheless, PRC and blood pressure are intricately related.