Our recent findings on molecular weight conversion in the case of renin are summarized and reviewed herein. The molecular weight of renin extracted from isolated renin granules from dogs and rats is approximately 40,000. This renin reacts with a constituent of kidney extract to form a high molecular weight renin of approximately 60,000. This constituent was termed a renin binding substance. This substance is probably contained in the cytosol of kidney cortex, and its characteristics are (1) heat instability, (2) non-dialyzability, (3) loss of binding ability by acidification at pH 3.0 and (4) apparent molecular weight is over 47,000. The binding reaction is mediated in two different ways; thiol oxidation with sodium tetrathionate and exposure of the reaction mixture at 0 degrees C for several days without thiol oxidation.