1. Kidney weight and content of protein, RNA and DNA were measured in rats with streptozotocin diabetes of varying duration. 2. Diabetic rats had larger kidneys than control rats: after 3 days of diabetes the weight increase was 15% and after 42 days of diabetes it was 90%. The protein content rose in parallel to the weight. 3. RNA content was already increased after 36 h of glycosuria, whereas DNA content was unchanged for the first 3 days of diabetes, and increased thereafter. The protein/DNA ratio increased rapidly during the first 3 days but remained constant thereafter. 4. Insulin treatment decreased the renal weight gain by about 67% during the first 8 days of diabetes, but did not prevent the increase in DNA. When insulin was started after 25 days of diabetes there was only a slight regression of kidney growth.