Urinary kallikrein is originated in the kidney. In this review it is postulated that urinary kallikrein excretion is related with the necessity to eliminate water. The principal releasing mechanism is a wash-out effect on renal kallikrein, produced by the flow reaching the tabular distal cell. Several systems can modify kallikrein excretion acting on the mechanisms of its release and/or synthesis. Urinary kallikrein regulates extracellular space inhibiting water distal reabsorption.