1. The initial blood pressure response to saralasin (Sar1-Ala8-angiotensin II) infusion was examined in 15 normal subjects, eight patients with untreated essential hypertension and 65 patients established on chronic haemogialysis (including six anephric patients), and related to measurements of plasma renin activity (PRA), angiotensin II, plasma catecholamines (noradrenaline and adrenaline), blood volume and extracellular fluid volume ([35S]sulphate space or exchangeable sodium). 2. A transient rise in arterial pressure, maximum after 5-6 min, occurred in all normal subjects, patient with essential hypertension and anephric patients, and in 41 of the 59 dialysis patients with kidneys. 3. In the normal subjects, saralasin infusion resulted in a significant rise in plasma noradrenaline (mean increase 360%, P less than 0-02) without change in plasma adrenaline concentration. The change in noradrenaline was significantly related to the change in mean blood pressure (P less than 0-05) and was similar to the response to 5 min of a 40 degree head-up tilt. 4. An increase in plasma noradrenaline also occurred in dialysis patients (P less than 0-005) but the change in mean blood pressure with saralasin in this group was inversely related to PRA (P less than 0-001) and angiotensin II (P less than 0-001), directly related to blood volume (P less than 0-001), but unrelated to the change in plasma noradrenaline. 5. The pressor response to saralasin may be mediated not only by angiotensin-like action on vascular receptors but also by an action on the central or peripheral autonomic nervous system.