The effect of four different homogenization methods, proteolytic enzymes and enzyme inhibitors on rat renal renin was studied. Rat kidney homogenate was treated with trypsin or pepsin. Neither of these enzymes had any effect on the molecular weight pattern. Enzyme inhibitors, 10 mM N-ethylmaleimide, 5.4 mM EDTA, 1.0 mM toluenesulfonyl fluoride, 2.3 mM o-phenanthroline, 2.3 mM p-hydroxymercuribenzoate, which was added to the homogenization medium, all or some of them, as well as 1/20 to the buffer solution used throughout the experiments for dialysis and column chromatography, did not alter the molecular weight pattern. These results suggest that renins, with molecular weights from above 60 000 Dalton to 37 000 Dalton were preformed in the rat kidney and extracted in a native form.