WE have found that human urinary kallikrein converts inactive plasma renin in vitro to an enzyme with renin-like activity. This observation is interesting because renal kallikrein, the presumed precursor of urinary kallikrein¹, is thought to be situated in or on cells of the distal convoluted tubule, including the macula densa cells^1–3, which are adjacent to the storage site of renin, the juxtaglomerular cells⁴. Also, human urinary kallikrein is a trypsin-like enzyme of highly restricted specificity⁵, and although trypsin is capable of activating inactive renin⁶, we have found that kallikrein is at least 10 times more active. Therefore it seems possible that urinary kallikrein is the in vivo converting enzyme of prorenin and may activate the renin–angiotensin–aldosterone hormonal system, as it participates in sodium and potassium homeostasis⁴ and in the blood pressure maintenance of a majority of hypertensive patients⁷.