1. A soluble fraction of renal cortical homogenate (cytosol) and renin extracted from isolated renin granules of the dog kidney were kept at 0 degrees C. 2. Although the molecular weight of the renin in the cytosol on day 1 was estimated to be about 40 000 by gel filtration, the renin was completely converted into a higher-molecular-weight form (60 000) by day 7. The renin in the granules kept its molecular size of 40 000 during cold storage. 3. This type of molecular-weight conversion could be performed without protease inhibitors. 4. Dithiothreitol neither inhibited the conversion into the higher-molecular-weight form of renin during cold storage nor led to a reduction in the molecular weight of renin, although the oxidation of thiol groups has been proposed as the mechanism for the molecular-weight conversion of renin. 5. Keeping a mixture of renin from the granules and crude renin-binding substance at 0 degrees C for 7 days resulted in the conversion of the renin into the higher-molecular-weight form, indicating that the renin-binding substance we have previously described is required for the conversion during cold storage. 6. Acidification caused the higher-molecular-weight form of renin formed in the cytosol to change to the lower-molecular-weight form, with a concomitant increase in activity of about 50%.