Active and inactive urinary kallikrein excretion rates were evaluated in 43 essential hypertensive men (45.4 +/- 5.6 yr) after normal-(120 mmol/day), low-(20 mmol/day), and high-(240 mmol/day) NaCl diets were given for 2 wk each. Patients were classified as salt-sensitive, salt-resistant, or counterregulating, on the basis of their blood pressure responses to the different NaCl intakes. Resulting data show that active and inactive kallikrein excretion rates were lower (P < 0.001) in salt-sensitive (active, 0.59 +/- 0.27 U/24 h; inactive, 3.45 +/- 1.31 U/24 h) than in salt-resistant (active, 1.41 +/- 0.35 U/24 h; inactive, 6.93 +/- 2.68 U/24 h) and in counterregulating hypertensive patients (active, 1.37 +/- 0.39 U/24 h; inactive, 6.32 +/- 2.58 U/24 h) after the normal NaCl diet. Salt-sensitive hypertensive patients showed also higher plasma digoxin-like substance (P < 0.001), atrial natriuretic peptide (P < 0.001), and fasting insulin (P < 0.005) levels than the other subgroups. Active kallikrein decreased after high and increased after low-NaCl intake in all groups. Inactive kallikrein varied similarly to active one in salt-resistant patients and counterregulating patients, whereas it increased during salt-loading in salt-sensitive patients. Consequently, the active/total kallikrein ratio decreased in salt-sensitive patients (from 20.2 +/- 3.5 to 5.82 +/- 1.02%, P < 0.05) when they switched from low- to high-NaCl intake, and the ratio was lower in these patients than in the other subgroups (P < 0.0001) after the high-NaCl diet. In conclusion, active and inactive kallikrein excretions after normal-NaCl intake are reduced in salt-sensitive hypertensive patients. The divergent active and inactive kallikrein responses to dietary NaCl changes in salt-sensitive patients could indicate an impairment of inactive to active kallikrein conversion during NaCl loading as a new mechanism in human salt-sensitive hypertension.