We measured urinary kallikrein by its esterolytic and kinin-forming activity in 5-minute urine samples obtained throughout continuous bleeding experiments in pigs to correlate possible changes in urinary kallikrein excretion during hemorrhagic hypotension with resin activity and renal cortical blood flow. Renin activity was determined in venous blood samples and renal cortical blood flow was estimated by the radiolabeled microsphere technique. The rate of urinary kallikrein excretion was increased about 4-fold within an arterial pressure range of 100-70 mm Hg, whereas below 70 mm Hg, arterial pressure urinary kallikrein activity declined to undetectable values. Renin activity was increased only 2-fold in the arterial pressure range between 100 and 70 mm Hg but was increased 4-fold at pressures below 70 mm Hg. The pressure range of 100 to 70 mm Hg corresponded to the autoregulation of renal cortical blood flow and glomerular filtration rate; below that pressure range, renal cortical blood flow dropped to about 10% of the initial value. Therefore, it seems that urinary kallikrein is activated mainly during the period of autoregulation, whereas renin activity is, in the main, increased below the autoregulatory range of pressure. The vasodilatory urinary kallikrein kinin system might be involved in maintaining sufficient local blood flow during autoregulation whereas a decrease in blood pressure below the autoregulatory range leads to a major increase in renin activity, thus illustrating the attempt of the organism to reestablish sufficient blood pressure to maintain autoregulation.