Primary hypertension is associated with a lack in renal kallikrein activity which might be one of the reasons for the blood pressure elevation. Some smaller and partially uncontrolled studies suggested that an oral substitution of glandular kallikrein lowers blood pressure by a kinin-mediated vasodilation and increased natriuresis. To test this hypothesis we treated in two studies over 100 patients with untreated mild to moderate primary hypertension (WHO I-II) for 5 resp. 12 weeks in a double blind randomized and placebo controlled manner with 1800 U glandular kallikrein orally. Blood pressure measurements were performed according to the two study designs after 3 and 5 resp. 8 and 12 weeks of treatment sphymomanometrically in the day time course. No significant changes in blood pressure by kallikrein treatment could be observed at any time. Neither renal kallikrein excretion, renin and ACE-activity nor blood glucose concentration in diabetics or non-diabetics was changed. Thus, we could undoubtedly demonstrate that oral applied glandular kallikrein has no effect on primary hypertension.