1. The behaviour of the potent vasoconstrictive endothelium-derived peptide endothelin-1 was evaluated in salt-sensitive hypertension. 2. Circulating and urinary endothelin-1 levels were evaluated in 30 men (mean age 44.6 +/- 3.1 years) with uncomplicated essential hypertension after three consecutive 2-week periods on an intermediate (120 mmol), low (20 mmol) and high (240 mmol) NaCl diet. On the same occasions, blood pressure was measured to identify salt-sensitive patients (n = 16), i.e. those patients showing a mean blood pressure increase > 10 mmHg when switching from a low to a high NaCl diet, and salt-resistant patients (n = 14), i.e. those who did not show such mean blood-pressure variations. 3. Plasma endothelin-1 levels were higher (P < 0.005) in salt-sensitive than in salt-resistant hypertensive patients after intermediate-, low- and high-NaCl diets. Urinary endothelin-1 excretion was similar in both groups after an intermediate-NaCl diet, whereas it was significantly higher in salt-sensitive than in salt-resistant hypertensive subjects after low (P < 0.002) and high (P < 0.007) NaCl diets. High NaCl intake induced a significant increase in plasma endothelin-1 levels (P < 0.002) as compared with intermediate and low NaCl diet levels in salt-sensitive patients, but did not in salt-resistant subjects. No significant NaCl intake-related variations of urinary endothelin-1 excretion were observed in either group. 4. Salt-sensitive hypertensives are characterized by increased levels of endothelin-1 in both plasma and urine. This fact suggests that blood-pressure sensitivity to NaCl intake could be associated with an increased risk of developing both renal and cardiovascular damage.