Administration of 1.5mg/kg of estriol intramuscularly and 15mg/kg of stilbestrol disulfate intraperitoneally daily for 15 days caused an increase in plasma renin substrate (PRS), accompanied by an increase in plasma renin activity (PRA) and a slight decrease in plasma renin concentration (PRC). Contrary to the slight suppres-sion of PRC, juxtaglomerular granulation index (JGI) was significantly increased in rats treated by estrogen. In the rats which developed hypertension by estrogen, PRA, PRC and JGI were a little higher than those in the rats which remained normotensive after the same estrogon treatment, but these differences were not statistically significant. Therefore, it seems rather difficult to attribute the development of estrogen hypertension only to the quantitative changes in the renin-angiotensin system.