An inactive kallikrein, which could be activated with trypsin was isolated from the rat kidney cortex using diethylaminoethyl (DEAE)-cellulose chromatography. The inactive kallikrein had no vasodilator action, whereas the injection of trypsin-activated form of this enzyme into the femoral artery of dogs resulted in a marked increase in the arterial blood flow. Apparent molecular weight of the inactive kallikrein was estimated to be 4.4 X 10(4) by gel filtration, and this enzyme was converted to the renal active kallikrein (M.W. 3.8 X 10(4] by trypsin. The inactive kallikrein is immunologically identical with the trypsin-activated form of inactive kallikrein and active kallikrein. There were no significant differences in the chromatographic behavior on a DEAE-cellulose column, Km value for prolyl-phenylalanylarginine-4-methylcoumaryl-7-amide hydrolysis and profile of inhibition by trypsin inhibitors between the active kallikrein and the trypsin-activated form of inactive kallikrein. The above properties of the renal inactive kallikrein were similar to those of inactive kallikrein found in the urine. These results suggest that the inactive kallikrein in the rat kidney would be proteolytically converted to its active enzyme and that a part of the inactive kallikrein would be excreted into urine in a form itself.