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.