1. Strains of spontaneously hypertensive and normotensive rats were selected by repeated inbreeding.
2. Brief ether anaesthesia was shown to produce a two- to three-fold increase in plasma renin activity in both strains.
3. Plasma renin activity was significantly higher in young spontaneously hypertensive than in normotensive rats of the same age (5–7 weeks). After the ninth week plasma renin activity decreased and, at week 45, became significantly lower in hypertensive than in normotensive rats.
4. When hypertension was established a significant inverse relationship was found between plasma renin activity and systolic blood pressure in spontaneously hypertensive and in normotensive rats.
5. It seems unlikely that the renin—angiotensin system plays a major role in the maintenance of the established spontaneous hypertension in this strain. However, renin hypersecretion may be important in the early pre-hypertensive stage of genetic hypertension in rats.