In order to verify the influence of salt sensitivity on the blood pressure response to orally administered kallikrein, we evaluated the efficacy of glandular kallikrein (derived from porcine pancreas) in 28 essential hypertensives (21 males and 9 females) aged between 40 and 62 years. After a placebo run-in period, the patients were assigned to receive oral kallikrein therapy (150 IU 3 times a day; n = 18 patients) or placebo (n = 10 patients) over a period of 8 days in a random double-blind fashion. In the salt-resistant patients (n = 8), kallikrein administration did not modify blood pressure levels. In the same group, natriuresis increased significantly after the treatment [from 94.51 ± 10.76 to Ill.65 ± 23.19 mEq/24 h (mmol/24 h), p < 0.039]. In the salt-sensitive patients (n = 10), blood pressure decreased with the kallikrein therapy (systolic: from 158.50 ± 9.20 to 144.50 ± 10.12 mm Hg, p < 0.005; diastolic: from 99.50 ± 2.16 to 90.0 ± 3.67 mm Hg, p < 0.024). In the same patients, urinary Na+ excretion increased considerably after the kallikrein treatment (from 101.07 ± 18.36 to 134.34 ± 18.27 mEq/ 24 h, p < 0.0001). Therefore, our data indicate that the oral kallikrein administration reduces blood pressure levels only in the salt-sensitive hypertensives. In both the salt-sensitive and the salt-resistant groups a marked increase in the 24-hour urinary excretion of sodium was observed after the kallikrein treatment. Therefore, the enhanced antihypertensive efficacy of the oral kallikrein administration observed in the sodium-sensitive patients is not only due to its natriuretic action but also to different kallikrein-related effects.