1. Sodium chloride solution (0.9%) or noradrenaline in doses of 4, 12 and 36 μgh −1 kg −1 was infused for five consecutive days, either intrarenally (by a new technique) or intravenously into rats with one kidney removed. 2. Intrarenal infusion of noradrenaline caused hypertension at doses which did not do so when infused intravenously. 3. Intrarenal compared with intravenous infusion of noradrenaline caused higher plasma noradrenaline concentrations and a shift of the plasma noradrenaline concentration-blood pressure effect curve towards lower plasma noradrenaline levels. 4. These results suggest that hypertension after chronic intrarenal noradrenaline infusion is produced by relatively higher levels of circulating noradrenaline and by triggering of an additional intrarenal pressor mechanism.

1. Baroreflex sensitivity was studied in relation to the development of spontaneous hypertension in rats (SH rats), with normotensive Wistar-Kyoto (WKY) rats as controls. Conscious, freely moving animals were studied at different times, ranging from 4 to 20 weeks after birth. 2. The youngest SH rats (4–6 weeks; n = 10) already had significantly ( P < 0.01) higher mean arterial blood pressure (112 ± 2 mmHg) than WKY rats of corresponding age (95 ± 4 mmHg; n = 10). Baroreflex sensitivity did not differ at this age (0.37 ± 0.04 vs 0.38 ± 0.05 ms/mmHg). 3. Mean arterial pressure increased rapidly in SH rats during further development, reaching a value of 166 ± 3 mmHg in 12–20 week old animals ( n = 25). In equally old WKY rats blood pressure was significantly ( P <0.001) lower (110 ± 6 mmHg, n = 25). Baroreflex sensitivity did not change during development of SH rats (0.40 ± 0.03 ms/mmHg in 12–20 weeks old SH rats), whereas it increased two- to three-fold in WKY rats (0.93 ± 0.08 ms/mmHg, P <0.001). 4. It is concluded that an increase in baroreflex sensitivity is part of the development of a normotensive cardiovascular system, whereas in SH rats responsiveness of the baroreceptor reflex remains depressed during the development and stabilization of hypertension.