Since the reduced kallikrein excretion demonstrated in essential hypertension suggested the possibility of an impairment in the renal kallikrein-kinin system, we decided to evaluate the efficacy and safety of oral kallikrein administration (glandular kallikrein derived from porcine pancreas) in 30 essential hypertensive subjects (21 males, 9 females, age range 34-62 years). Twenty subjects took 150 IU kallikrein t.i.d. for eight days; during this period their sodium intake remained normal (120 mEq Na+/die). Ten subjects took placebo. After the trial period, urinary kallikrein in the active group increased from 0.9 +/- 0.4 U/24 h (normal value greater than 1.2 U/24 h) to 1.6 +/- 1 U/24 h (p less than 0.05); systolic and diastolic blood pressure decreased respectively from 154.6 +/- 13.8 mmHg to 140.3 +/- 12.5 mmHg (p less than 0.01) and from 92.5 +/- 1.5 mmHg to 86 +/- 3.9 mmHg (p less than 0.025); urinary sodium and potassium excretion increased respectively from 96.7 +/- 17 mEq/24 h to 119.1 +/- 32.3 mEq/24 h (p less than 0.05) and from 36.7 +/- 11 mEq/24 h to 43.5 +/- 12.8 mEq/24 h (p less than 0.05). One patient in the kallikrein group suffered a transient episode of gastric pain. No modifications of the parameters evaluated were observed in the placebo group. We conclude that kallikrein has a mild hypotensive effect in hypertensive subjects and is generally well-tolerated. Its antihypertensive effect is probably due to the sodiuretic action of the substance.